Material and method for colorimetric detection of small-molecule targets

ABSTRACT

The subject invention provides methods, assays, and products for visual detection of small-molecule targets in a sample in both clinical and field settings within minutes. The subject invention is based on an aptamer sensor that reports the presence of small-molecule target via a sensitive colorimetric signal for naked-eye detection. The aptamer sensor is a CBSAzyme-based sensor having both target-mediated cooperative behavior of the CBSA and peroxidase-mimicking catalytic activity of DNAzyme. The subject invention also provides methods of using the CBSAzyme-based sensor.

GOVERNMENT SUPPORT

This invention was made with government support under DA036821 awardedby the National Institutes of Health. The government has certain rightsin the invention.

SEQUENCE LISTING

The Sequence Listing for this application is labeled“SeqList-02Nov18-ST25.txt,” which was created on Nov. 2, 2018, and is 8KB. The Sequence Listing is incorporated herein by reference in itsentirety.

BACKGROUND OF THE INVENTION

Small molecules are important targets with the potential of clinical orcommercial applications such as medical diagnostics, environmentalmonitoring, and forensic science. Thus, efforts to develop methods forportable, low-cost, and quantitative on-site detection of a broad rangeof small molecules are gaining momentum.

Cocaine is a central nervous system stimulant that increases levels ofdopamine and potently inhibits neurotransmitter reuptake at the synapse.Abuse of cocaine has been shown to cause anxiety, paranoia, mooddisturbances, organ damage, and violent behavior.

Synthetic cathinones (also known as bath salts) are designer drugssharing a similar core structure with amphetamines and3,4-methylenedioxy-methamphetamine (MDMA). They are highly addictivecentral nervous system stimulants, and are associated with many negativehealth consequences, including even death. Although these drugs haveemerged only recently, abuse of bath salts has become a threat to publichealth and safety due to their severe toxicity, increasingly broadavailability, and difficulty of regulation. More importantly, there iscurrently no reliable presumptive test for any synthetic cathinone.Chemical spot tests used to detect conventional drugs such as cocaine,methamphetamine, and opioids show no cross-reactivity to syntheticcathinones.

Methods that are highly sensitive and selective, includinghigh-performance liquid chromatography (HPLC) and gaschromatography-mass spectrometry (GC-MS), have been used for thedetection of small molecules. However, these methods are time-consumingand require expensive reagents, advanced equipment, complex samplepreparation, and/or trained operators.

Various immunoassays have also been developed for the detection of smallmolecules such as cocaine and/or its major metabolite benzoylecgonine inbiofluids, including the enzyme-linked immunosorbent assay (ELISA).Unfortunately, the use of immunoassays for the detection of designerdrugs is often limited because of the high cost of generating newantibodies and issues with narrow target binding-spectrum and poorspecificity.

Nucleic acid-based bioaffinity elements, known as aptamers, can beisolated in vitro through systematic evolution of ligands by exponentialenrichment (SELEX) processes to bind various targets with highspecificity and affinity, including proteins, metal ions, smallmolecules, and even whole cells. They have gained considerable attentionas bio-recognition elements with diverse applications in areas such asdrug screening, medical diagnostics, and environmental monitoring. Thisis in part because aptamers are chemically stable, offering long shelflives, and can be synthesized at a low cost with high reproducibility.Also, aptamers can be engineered to have tunable target-bindingaffinities or various functionalities. These advantages make aptamersideal for use in biosensors.

Among the numerous aptamer-based sensing platforms, colorimetric assaysare especially desirable for on-site detection, as they can beinterpreted by the naked eye and do not require any specializedequipment to obtain a readout. For example, gold nanoparticles (AuNPs)have been widely employed with aptamers as sensitive colorimetric signalreporters for naked-eye small-molecule detection.G-quadruplex-structured DNA enzymes (DNAzymes) are alternative signalreporters for colorimetric aptamer-based assays. However, most of theseassays offer only limited capabilities for naked-eye detection, becausethe resulting absorbance changes can only be detected by instruments.

Therefore, there is a need for methods and materials for rapid,naked-eye small-molecule detection. Assays using such materials andmethods provide essential features such as ease of use,cost-effectiveness, rapid turnaround time and superior sensitivity andspecificity.

BRIEF SUMMARY OF THE INVENTION

The subject invention provides methods, assays, and products for rapid,naked-eye detection of small molecules in a sample, in particular, inboth clinical and field settings. The subject invention is based on anaptamer sensor that reports the presence of small-molecule target suchas cocaine and synthetic cathinones via a sensitive colorimetric signalfor naked-eye detection. Specifically, exemplified herein is a methodfor detecting cocaine and synthetic cathinones.

In one embodiment, the aptamer sensor is a CBSAzyme-based sensor havingboth target-mediated cooperative behavior of the CBSA andperoxidase-mimicking catalytic activity of DNAzyme.

The CBSAzyme-based sensor comprises a long fragment and a shortfragment, the long fragment comprising a first segment of a splitDNAzyme and a long fragment of a CBSA, the short fragment comprising asecond segment of the split DNAzyme and a short fragment of the CBSA.The two fragments of the CBSAzyme remain separate in the absence of thesmall-molecule target, but effectively assemble in the presence of thesmall-molecule target. The assembly of the two fragments of the CBSAzymeactivates the DNAzyme that subsequently catalyzes the oxidation of2,2′-azinobis(3-ethylbenzthiazo-line)-6-sulfonic acid (ABTS), producinga visible color change from colorless to dark green that reveals thepresence of the target within minutes.

The subject invention also provides methods of using the CBSAzyme-basedassays for rapid and naked-eye detection of small-molecule targets in asample. The method comprises contacting the sample with a CBSAzyme-basedsensor selective for the small-molecule target and detecting thesmall-molecule target in the sample by measuring a signal generated froma signal reporter. In a specific embodiment, the signal reporter isABTS, and the signal generated from the signal reporter is a colorchange resulting from the H₂O₂-mediated oxidation of ABTS to ABTS^(•+)by the peroxidase-mimicking catalytic activity of the assembled splitDNAzyme. Thus, a change in color is indicative of the presence andquantity of the small-molecule target in the sample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C show optimizing the activity of a splitG-quadruplex-structured DNAzyme using a duplex DNA template. (1A)Structures of hemin, ABTS, and the duplex DNA-DNAzyme conjugate formedby strands A (SEQ ID NO: 1) and B (SEQ ID NO: 5) (left) and the schemefor DNAzyme-mediated oxidation of ABTS to produce the green-coloredABTS^(•+) (right). X, Y and Z indicate selected modification siteswithin the spacers between GGG repeats. (1B, 1C) (left) Structures ofthe assembled split DNAzyme modules of (1B) A0 (SEQ ID NO: 38)—B0 (SEQID NO: 41), A1 (SEQ ID NO: 39)—B0 (SEQ ID NO: 41), A2 (SEQ ID NO: 40)—B0(SEQ ID NO: 41), (1C) A2 (SEQ ID NO: 40)—B0 (SEQ ID NO: 41), and A2 (SEQID NO: 40)—B1 (SEQ ID NO: 42) and (right) reaction rates of each DNAzymein terms of nanomolar ABTS^(•+) produced per second.

FIGS. 2A-2B show the optimization of the catalytic activity of anassembled split DNAzyme tethered to a DNA duplex. Time-coursemeasurement of absorbance at 418 nm using split DNAzymes with (2A)strand B0 and different variants of strand A and (2B) strand A2 anddifferent variants of strand B. Hemin alone was used as a control.

FIGS. 3A-3B show the strategy for engineering the cocaine-bindingCBSAzyme from a cocaine-binding CBSA. The split DNAzyme segments fromstrands A2 and B1 are conjugated to the ends of COC-5335 (LF: SEQ ID NO:8; SF: SEQ ID NO: 9) (3A) to generate CBSAzyme-5335-22 (LF: SEQ ID NO:10; SF: SEQ ID NO: 11) (3B).

FIG. 4 shows the working principle of colorimetric small-moleculedetection based on target-induced CBSAzyme assembly and oxidation ofcolorless ABTS (left) to produce green-colored ABTS^(•+) (right). Theblack- and red-colored segments are the long and short fragments of theCBSAzyme, respectively, while blue- and pink-colored segments representthe linker and split DNAzyme segments, respectively.

FIGS. 5A-5C show the effect of CBSA stem-length on the performance ofthe cocaine-detecting CBSAzyme. Time-course of absorbance measurementsfor (5A) CBSAzyme-5335-22 (LF: SEQ ID NO: 10; SF: SEQ ID NO: 11), (5B)CBSAzyme-5334-22 (LF: SEQ ID NO:12; SF: SEQ ID NO: 13), and (5C)CBSAzyme-5333-22 (LF: SEQ ID NO:14; SF: SEQ ID NO: 15) in the presenceand absence of 250 μM cocaine. [Each fragment]=0.25 μM

FIGS. 6A-6C show the effect of DNAzyme split mode on enzyme catalyticactivity. (6A) Constructing the 1:3 mode split DNAzyme and subsequentconjugation to COC-5334 via dinucleotide linkers to generateCBSAzyme-5334-13 (LF: SEQ ID NO: 18; SF: SEQ ID NO: 19). Structures andtime-course absorbance measurements for (6B) CBSAzyme-5334-13 and (6C)CBSAzyme-5334-22 in the absence and presence of 250 μM cocaine. [Eachfragment]=1 μM.

FIGS. 7A-7B show the determination of the enzyme kinetics ofCBSAzyme-5334-22 and CBSAzyme-5334-13. (7A) The initial reaction rate ofthe CBSAzymes was plotted against the concentration of ABTS. (7B) TheMichaelis-Menten constant (K_(M)) and turnover number (k_(cat)) in thepresence and absence of cocaine were obtained from the plotted curves.

FIG. 8 shows the circular dichroism spectra of 1 μM CBSAzyme-5334-13alone as well as with 250 μM cocaine, 1 μM hemin, or both. Circulardichroism contributions from cocaine and hemin were subtracted. Bufferconditions: 40 mM HEPES (pH 7.0), 2 mM KCl, 28 mM NaCl, 1% (v/v) DMSO.

FIG. 9 shows the circular dichroism spectra of 1 μM CBSAzyme-5334-22alone as well as with 250 μM cocaine, 1 μM hemin, or both. Circulardichroism contributions from cocaine and hemin were subtracted. Bufferconditions: 40 mM HEPES (pH 7.0), 7 mM KCl, 7 mM NaCl, 1% DMSO.

FIGS. 10A-10C show the optimization of the linker of CBSAzyme-5334-13.Structure and time-course of CBSAzyme-5334-13 with linkers comprising(10A) AT/AA (LF: SEQ ID NO: 18; SF: SEQ ID NO: 19), (10B) A/AA (LF: SEQID NO: 20; SF: SEQ ID NO: 19), and (10C) A/A (LF: SEQ ID NO: 20; SF: SEQID NO: 21) in the absence and presence of 250 μM cocaine. The absorbanceat 418 nm was recorded over 30 minutes. [Each fragment]=1 μM.

FIG. 11 shows the circular dichroism spectra of 1 μM COC-CBSAzyme (LF:SEQ ID NO: 20; SF: SEQ ID NO: 19) alone as well as with 250 μM cocaine,1 μM hemin, or both. Circular dichroism contributions from cocaine andhemin were subtracted. Buffer conditions: 40 mM HEPES (pH 7.0), 1 mMKCl, 30 mM NaCl, 1% DMSO.

FIGS. 12A-12C show utilizing COC-CBSAzyme for the visual detection ofcocaine. (12A) Time-dependent absorbance change at 418 nm with (1)reaction buffer alone, (2) hemin alone, (3) short fragment with hemin,(4) long fragment with hemin, and both fragments plus hemin in the (5)absence or (6) presence of cocaine. (12B) Calibration curve generatedusing 0-1,000 μM cocaine. Inset represents linear range from 0 to 10 Mcocaine. (12C) Photographs of samples containing 0-1,000 μM cocaineafter 15 minutes of reaction. [Each fragment]=1 μM.

FIG. 13 shows utilizing COC-CBSAzyme for the naked-eye detection ofcocaine. Experimental setup shows sample contents and photographs depictthe color of the samples containing (1) reaction buffer alone, (2) 1 μMhemin alone, (3) 1 μM short fragment with hemin, (4) 1 μM long fragmentwith hemin, and both fragments plus hemin in the (5) absence and (6)presence of 250 μM cocaine after 15 minutes of reaction.

FIGS. 14A-14B show the optimization of the concentrations of the longand short fragments of COC-CBSAzyme in the CBSAzyme-based colorimetricassay. The assay was performed in the presence and absence of 250 μMcocaine with (14A) 1 μM long fragment and 0.2, 0.5 1, 1.5 or 2 μM ofshort fragment, or (14B) 0.2, 0.5 1, 1.5 or 2 μM long fragment and 1 μMof short fragment. Top panels show the signal gain obtained with variousratios of long and short fragment after 15 minutes of reaction. Bottompanels are photographs depicting the color of the samples.

FIG. 15 shows the time-course visual detection of cocaine usingCOC-CBSAzyme (LF: SEQ ID NO: 20; SF: SEQ ID NO: 19). Photographs of thesamples containing different concentrations of cocaine (0-300 μM) atdifferent time points (0-30 min) are shown. The color of thecocaine-containing samples progressively changes over time, while thecolor of the cocaine-free sample only changed slightly.

FIGS. 16A-16B show the comparison of the target-responsiveness ofCOC-CBSAzyme (LF: SEQ ID NO: 20; SF: SEQ ID NO: 19) and SAzyme-334 (LF:SEQ ID NO: 22; SF: SEQ ID NO: 23). (16A) Structures of COC-CBSAzyme andSAzyme-334. (16B) Photographs of samples containing COC-CBSAzyme andSAzyme-334 in the presence of various concentrations of cocaine (0-1,000μM) after 2 minutes (top panel) and 15 minutes (bottom panel) ofreaction.

FIGS. 17A-17C show the specificity of the CBSAzyme-based assay forcocaine detection. (17A) Chemical structures of interferents tested inthe assay. (17B) Absorbance of samples at 418 nm after 15 minutes ofreaction containing various interferents (250 μM). (17C) Photographs ofsamples containing various interferents (250 μM) after 15 minutes ofreaction.

FIGS. 18A-18C show the engineering and characterization of anMDPV-binding CBSA. (18A) An isolated MDPV-binding aptamer (SEQ ID NO:24) (I) was used to generate a pair of split aptamers with destabilizedstems (II), which were then merged to create MDPV-6335 (LF: SEQ ID NO:25; SF: SEQ ID NO: 26) (III). (18B) Scheme of a MDPV fluorescence assayusing a fluorophore-quencher modified version of MDPV-6335 (LF: SEQ IDNO: 25; SF: SEQ ID NO: 27). (18C) Binding curve of MDPV-6335 generatedusing 0-3,000 μM MDPV and the calculated values for K_(1/2) and n_(H)using the Hill Equation.

FIGS. 19A-19B show the design and performance of an MDPV-bindingCBSAzyme (LF: SEQ ID NO: 28; SF: SEQ ID NO: 29). (19A) Engineering ofMDPV-CBSAzyme using MDPV-6335 and the optimized split DNAzyme fragments(SEQ ID NO: 44 and SEQ ID NO: 45). In the presence of MDPV, the twoCBSAzyme fragments assemble, such that the DNAzyme can catalyze theoxidation of ABTS to generate the green-colored ABTS^(•+). (19B)Time-course absorbance measurement at 418 nm with reaction buffercontaining 1 μM MDPV-CBSAzyme in the presence and absence of 250 μMMDPV.

FIG. 20 shows the circular dichroism spectra of 1 μM MDPV-CBSAzyme (LF:SEQ ID NO: 28; SF: SEQ ID NO: 29) alone and with 200 μM MDPV, 1 μMhemin, or both. Circular dichroism contributions from MDPV and heminwere subtracted. Buffer conditions: 40 mM HEPES pH 7, 7 mM KCl, 77 mMNaCl, 1% DMSO.

FIGS. 21A-21C show utilizing MDPV-CBSAzyme for the visual detection ofMDPV. (21A) Time-dependent absorbance change at 418 nm with (1) reactionbuffer alone, (2) hemin alone, (3) the short fragment plus hemin, (4)the long fragment with hemin, and both fragments with hemin in the (5)absence or (6) presence of MDPV. (21B) Photographs of samples containingdifferent concentrations of MDPV (0 to 1,000 μM) after 15 minutes ofreaction. (21C) Assay calibration curve with MDPV concentrations rangingfrom 0 to 1,000 μM. Inset represents linear range from 0 to 10 μM. [Eachfragment]=1 μM

FIG. 22 shows utilizing MDPV-CBSAzyme for the naked-eye detection ofMDPV. Experimental setup shows sample contents and photographs depictthe color of the samples containing (1) reaction buffer alone, (2) 1 μMhemin alone, (3) 1 μM short fragment with hemin, (4) 1 μM long fragmentwith hemin, and both fragments plus hemin in the (5) absence and (6)presence of 250 μM MDPV after 15 minutes of reaction.

FIG. 23 shows the time-course visual detection of MDPV usingMDPV-CBSAzyme. Photographs of the samples containing differentconcentrations of MDPV (0-300 μM) at different time points (0-30 min)are shown. The color of the MDPV-containing samples progressivelychanges over time, while the color of the cocaine-free sample onlychanged slightly.

FIGS. 24A-24B show the cross-reactivity and specificity of thecolorimetric MDPV-CBSAzyme-based assay. Photographs of samplescontaining (24A) buffer-only control sample, 11 synthetic cathinones and(24B) MDPV or 5 interferents after 15 minutes of reaction. All drugs orinterferents were present at 250 μM. [Each fragment]=1 μM.

FIG. 25 shows the target-cross-reactivity of the MDPV-CBSAzyme-basedassay. Absorbance of samples at 418 nm after 15 minutes for samplescontaining 11 different synthetic cathinones at a concentration of 250μM is shown.

FIG. 26 shows the specificity of the MDPV-CBSAzyme-based assay.Absorbance of samples at 418 nm after 15 minutes for samples containingvarious interferents (250 μM) is shown. MDPV is used as a positivecontrol.

FIG. 27 shows the time-course for visual detection of cocaine in 10%saliva using COC-CBSAzyme (LF: SEQ ID NO: 20; SF: SEQ ID NO: 19). (left)Photographs of the samples containing different concentrations ofcocaine (COC) (0-1000 μM) at different time points (0-5 min) are shown.The color of the cocaine-containing samples progressively changes overtime, while the color of the cocaine-free sample only changed slightly.(right) Calibration curve generated using 0-1000 μM cocaine after 5minutes of reaction. The limit of visual detection is 3 μM cocaine.

FIG. 28 shows the time-course visual detection of cocaine in 10% urineusing COC-CBSAzyme (LF: SEQ ID NO: 20; SF: SEQ ID NO: 19). (left)Photographs of the samples containing different concentrations ofcocaine (COC) (0-1000 μM) in at different time points (0-5 min) areshown. The color of the cocaine-containing samples progressively changesover time, while the color of the cocaine-free sample only changedslightly. (right) Calibration curve generated using 0-1000 μM cocaineafter 5 minutes of reaction. The limit of visual detection is 3 JAMcocaine.

FIG. 29 shows the time-course visual detection of MDPV in 50% salivausing MDPV-CBSAzyme (LF: SEQ ID NO: 28; SF: SEQ ID NO: 29). (left)Photographs of the samples containing different concentrations of MDPV(0-1000 μM) at different time points (0-30 min) are shown. The color ofthe MDPV-containing samples progressively changes over time, while thecolor of the MDPV-free sample only changed slightly. (right) Calibrationcurve generated using 0-1000 μM MDPV after 15 minutes of reaction. Thelimit of visual detection is 30 μM MDPV.

FIG. 30 shows the time-course visual detection of MDPV in 50% urineusing MDPV-CBSAzyme (LF: SEQ ID NO: 28; SF: SEQ ID NO: 29). (left)Photographs of the samples containing different concentrations of MDPV(0-1000 μM) at different time points (0-30 min) are shown. The color ofthe MDPV-containing samples progressively changes over time, while thecolor of the MDPV-free sample only changed slightly. (right) Calibrationcurve generated using 0-1,000 μM MDPV after 15 minutes of reaction. Thelimit of visual detection is 30 μM MDPV.

BRIEF DESCRIPTION OF SEQUENCES

SEQ ID NOs: 1-7 are sequences of duplex DNA-DNAzyme conjugatescontemplated for use according to the subject invention.

SEQ ID NO: 8 is the DNA sequence of the long fragment of aptamerCOC-5335 contemplated for use according to the subject invention.

SEQ ID NO: 9 is the DNA sequence of the short fragment of aptamerCOC-5335 contemplated for use according to the subject invention.

SEQ ID NO: 10 is the DNA sequence of the long fragment ofCOC-CBSAzyme-5335-22 contemplated for use according to the subjectinvention.

SEQ ID NO: 11 is the DNA sequence of the short fragment ofCOC-CBSAzyme-5335-22 contemplated for use according to the subjectinvention.

SEQ ID NO: 12 is the DNA sequence of the long fragment ofCOC-CBSAzyme-5334-22 contemplated for use according to the subjectinvention.

SEQ ID NO: 13 is the DNA sequence of the short fragment ofCOC-CBSAzyme-5334-22 contemplated for use according to the subjectinvention.

SEQ ID NO: 14 is the DNA sequence of the long fragment ofCOC-CBSAzyme-5333-22 contemplated for use according to the subjectinvention.

SEQ ID NO: 15 is the DNA sequence of the short fragment ofCOC-CBSAzyme-5333-22 contemplated for use according to the subjectinvention.

SEQ ID NO: 16 is the DNA sequence of the short fragment of aptamerCOC-5334 contemplated for use according to the subject invention.

SEQ ID NO: 17 is the DNA sequence of the long fragment of aptamerCOC-5334 contemplated for use according to the subject invention.

SEQ ID NO: 18 is the DNA sequence of a long fragment ofCOC-CBSAzyme-5334-13 with a AT linker contemplated for use according tothe subject invention.

SEQ ID NO: 19 is the DNA sequence of a short fragment ofCOC-CBSAzyme-5334-13 with a AA linker contemplated for use according tothe subject invention.

SEQ ID NO: 20 is the DNA sequence of a long fragment ofCOC-CBSAzyme-5334-13 with a A linker contemplated for use according tothe subject invention.

SEQ ID NO: 21 is the DNA sequence of a short fragment ofCOC-CBSAzyme-5334-13 with a A linker contemplated for use according tothe subject invention.

SEQ ID NO: 22 is the DNA sequence of the long fragment of SAzyme-334contemplated for use according to the subject invention.

SEQ ID NO: 23 is the DNA sequence of the short fragment of SAzyme-334contemplated for use according to the subject invention.

SEQ ID NO: 24 is the DNA sequence of a MDPV-binding aptamer contemplatedfor use according to the subject invention.

SEQ ID NO: 25 is the DNA sequence of the long fragment of MDPV-6335contemplated for use according to the subject invention.

SEQ ID NO: 26 is the DNA sequence of a short fragment of MDPV-6335contemplated for use according to the subject invention.

SEQ ID NO: 27 is the DNA sequence of a short fragment of MDPV-6335 withfluorephore-quencher modifications contemplated for use according to thesubject invention.

SEQ ID NO: 28 is the DNA sequence of a long fragment of MDPV-CBSAzymecontemplated for use according to the subject invention.

SEQ ID NO: 29 is the DNA sequence of a short fragment of MDPV-CBSAzymecontemplated for use according to the subject invention.

SEQ ID NO: 30 is the DNA sequence of a long fragment of a parent splitMDPV aptamer contemplated for use according to the subject invention.

SEQ ID NO: 31 is the DNA sequence of a short fragment of a parent splitMDPV aptamer contemplated for use according to the subject invention.

SEQ ID NO: 32 is the DNA sequence of a long fragment of a parent splitMDPV aptamer contemplated for use according to the subject invention.

SEQ ID NO: 33 is the DNA sequence of a short fragment of a parent splitMDPV aptamer contemplated for use according to the subject invention.

SEQ ID NO: 34 is the DNA sequence of the long fragment of COC-5333contemplated for use according to the subject invention.

SEQ ID NO: 35 is the DNA sequence of the short fragment of COC-5333contemplated for use according to the subject invention.

SEQ ID NOs: 36-42 are the sequences of fragments of the split DNAzymecontemplated for use according to the subject invention.

SEQ ID NO: 43 is the sequence of a DNAzyme contemplated for useaccording to the subject invention.

SEQ ID NOs: 44-45 are DNA sequences of segments of a split DNAzymecontemplated for use according to the subject invention.

DETAILED DESCRIPTION OF THE INVENTION

The subject invention provides methods, assays, and products for rapid,naked-eye detection of small molecules in a sample, in particular, inboth clinical and field settings. In one embodiment, the sample is abiological sample of a subject. In specific embodiments, the biologicalsample is selected from blood, plasma, urine, tears, sweat and saliva.The subject may be any animal or human, preferably, a human. The subjectmay also be any animal including, but not limited to, non-humanprimates, rodents, dogs, cats, horses, cattle, pigs, sheep, goats,chickens, guinea pigs, hamsters and the like.

In one embodiment, the sample is an environmental sample, for example,water, soil, air, or plant sample. In another embodiment, the sample isa seized drug sample, for instance, a street drug sample seized by lawenforcement or government officials.

Small Molecules

The term “target,” “small molecule,” or “small-molecule target,” as usedherein, includes any molecule capable of being detected using an aptamertechnique. In certain embodiments, the small molecule has a molecularweight less than 1000 Daltons, less than 900 Daltons, less than 800Daltons, less than 700 Daltons, less than 600 Daltons, less than 500Daltons, less than 400 Daltons, less than 300 Daltons, or less than 200Daltons.

In specific embodiments, the small-molecule target may be an amino acid,an amino acid-related molecule, a peptide, a steroid, a lipid, a sugar,a carbohydrate, a biomarker, a drug molecule, a drug metabolite, acoenzyme, a nucleotide (nt), a nucleotide-related molecule, a pyridinenucleotide, a cyclic nucleotide, or a cyclic dinucleotide. In otherembodiments, the small-molecule target may be an infective agent,antigen, toxin, disease biomarker, or a specific metal ion.

In one embodiment, the small molecule is a drug molecule. In specificembodiments, the drug molecule is cocaine or a cocainederivative/metabolite. The cocaine derivative may or may not have thecore structure of cocaine. Exemplary cocaine derivatives/metabolitesinclude, but are not limited to, 4-fluorococaine, 2-hydroxycocaine,3-(p-fluorobenzoyloxy)tropane (pFBT), cocaethylene, norcocaine,procaine, and dimethocaine.

In one embodiment, the drug molecule is a cathinone, a cathinonederivative, or synthetic cathinone, such as a ring-substituted cathinonederivative or synthetic cathinone. The synthetic cathinone has a generalstructure of formula (I)

wherein R₁ and R₂, are each independently selected from the groupconsisting of hydrogen, alkyl, aryl, heteroaryl, cycloalkyl,cycloalkenyl, heterocycloalkyl, alkenyl, alkynyl, haloalkyl, acyl,alkoxy, halogen, and hydroxylalkyl; R₃ is hydrogen or alkyl. R₄, and R₅are each independently selected from the group consisting of hydrogen,alkyl, aryl, heteroaryl, cycloalkyl, cycloalkenyl, heterocycloalkyl,alkenyl, alkynyl, haloalkyl, acyl, halogen, and hydroxylalkyl; and R₆ ishydrogen or alkyl.

In a further embodiment, R₁ and R₂ are independently a halogen such asfluorine, chlorine, bromine or iodine.

In some embodiments, R₁ and R₂, taken together with the carbon atoms towhich they are attached, form a substituted or unsubstituted 5- or6-membered homocyclic or heterocyclic ring. For example, R₁ and R₂ mayform a methylenedioxy group or aromatic group such as a benzene ring.

In other embodiments, R₄ and R₅, taken together with the nitrogen atomto which they are attached, form a substituted or unsubstituted 5- or6-membered heterocyclic ring. For example, R₄ and R₅ may form apyrrolidino group.

As used herein, “alkyl” means linear saturated monovalent radicals of atleast one carbon atom or a branched saturated monovalent of at leastthree carbon atoms. It may include hydrocarbon radicals of at least onecarbon atom, which may be linear. Examples include, but are not limitedto, methyl, ethyl, propyl, 2-propyl, n-butyl, iso-butyl, tert-butyl,pentyl, hexyl, and the like.

As used herein, “acyl” means a radical —C(O)R where R includes, but isnot limited to, hydrogen, alkyl or cycloalkyl, and heterocycloalkyl.Examples include, but are not limited to, formyl, acetyl, ethylcarbonyl,and the like. An aryl group may be substituted or unsubstituted.

As used herein, “alkylamino” means a radical —NHR or —NR2 where each Ris, independently, an alkyl group. Examples include, but are not limitedto, methylamino, (1-methylethyl)amino, dimethyl amino, methylethylamino,di(1-methylethyl)amino, and the like. An alkylamino may be substitutedor unsubstituted.

As used herein, “hydroxyalkyl” means an alkyl radical substituted withone or more hydroxy groups. Representative examples include, but are notlimited to, hydroxymethyl, 2-hydroxyethyl; 2-hydroxypropyl;3-hydroxypropyl; 1-(hydroxymethyl)-2-methylpropyl; 2-hydroxybutyl;3-hydroxybutyl; 4-hydroxybutyl; 2,3-dihydroxypropyl;2-hydroxy-1-hydroxymethylethyl; 2,3-dihydroxybutyl; 3,4-dihydroxybutyland 2-(hydroxymethyl)-3-hydroxy-propyl; preferably 2-hydroxyethyl;2,3-dihydroxypropyl and 1-(hydroxymethyl) 2-hydroxyethyl. A hydroxyalkylmay be substituted or unsubstituted.

As used herein, “alkenyl” refers to a straight or branched hydrocarbonchain containing one or more double bonds. The alkenyl group may have 2to 9 carbon atoms, although the present definition also covers theoccurrence of the term “alkenyl” where no numerical range is designated.The alkenyl group may also be a medium size alkenyl having 2 to 9 carbonatoms. The alkenyl group could also be a lower alkenyl having 2 to 4carbon atoms. The alkenyl group may be designated as “C₂₋₄ alkenyl” orsimilar designations. By way of example only, “C₂₋₄ alkenyl” indicatesthat there are two to four carbon atoms in the alkenyl chain, i.e., thealkenyl chain is selected from ethenyl; propen-1-yl; propen-2-yl;propen-3-yl; buten-1-yl; buten-2-yl; buten-3-yl; buten-4-yl;1-methyl-propen-1-yl; 2-methyl-propen-1-yl; 1-ethyl-ethen-1-yl;2-methyl-propen-3-yl; buta-1,3-dienyl; buta-1,2,-dienyl andbuta-1,2-dien-4-yl. Typical alkenyl groups include, but are in no waylimited to, ethenyl, propenyl, butenyl, pentenyl, and hexenyl, and thelike.

As used herein, “alkynyl” refers to a straight or branched hydrocarbonchain comprising one or more triple bonds. The alkynyl group may have 2to 9 carbon atoms, although the present definition also covers theoccurrence of the term “alkynyl” where no numerical range is designated.The alkynyl group may also be a medium size alkynyl having 2 to 9 carbonatoms. The alkynyl group could also be a lower alkynyl having 2 to 4carbon atoms. The alkynyl group may be designated as “C₂₋₄ alkynyl” orsimilar designations. By way of example only, “C₂₋₄ alkynyl” indicatesthat there are two to four carbon atoms in the alkynyl chain, e.g., thealkynyl chain is selected from ethynyl, propyn-1-yl, propyn-2-yl,butyn-1-yl, butyn-3-yl, butyn-4-yl, and 2-butynyl. Typical alkynylgroups include, but are in no way limited to, ethynyl, propynyl,butynyl, pentynyl, and hexynyl, and the like.

As used herein, “cycloalkyl” means a fully saturated carbocyclyl ring orring system. Examples include, but are not limited to, cyclopropyl,cyclobutyl, cyclopentyl, and cyclohexyl.

As used herein, “aryl” refers to a carbocyclic (all carbon) monocyclicor multicyclic aromatic ring system (including fused ring systems wheretwo carbocyclic rings share a chemical bond). The number of carbon atomsin an aryl group can vary. For example, the aryl group can be a C₆-C₁₄aryl group, a C₆-C₁₀ aryl group, or a C₆ aryl group. Examples of arylgroups include, but are not limited to, phenyl, benzyl, α-naphthyl,β-naphthyl, biphenyl, anthryl, tetrahydronaphthyl, fluorenyl, indanyl,biphenylenyl, and acenaphthenyl. Preferred aryl groups are phenyl andnaphthyl.

As used herein, “heteroaryl” refers to an aromatic ring or ring system(i.e., two or more fused rings that share two adjacent atoms) thatcomprise(s) one or more heteroatoms, that is, an element other thancarbon, including but not limited to, nitrogen, oxygen and sulfur, inthe ring backbone. When the heteroaryl is a ring system, every ring inthe system is aromatic. The heteroaryl group may have 5-18 ring members(i.e., the number of atoms making up the ring backbone, including carbonatoms and heteroatoms), although the present definition also covers theoccurrence of the term “heteroaryl” where no numerical range isdesignated. Examples of heteroaryl rings include, but are not limitedto, furyl, thienyl, phthalazinyl, pyrrolyl, oxazolyl, thiazolyl,imidazolyl, pyrazolyl, isoxazolyl, isothiazolyl, triazolyl,thiadiazolyl, pyridinyl, pyridazinyl, pyrimidinyl, pyrazinyl, triazinyl,quinolinyl, isoquinlinyl, benzimidazolyl, benzoxazolyl, benzothiazolyl,indolyl, isoindolyl, and benzothienyl.

As used herein, “haloalkyl” refers to an alkyl group, in which one ormore of the hydrogen atoms are replaced by a halogen (e.g.,mono-haloalkyl, di-haloalkyl and tri-haloalkyl). Such groups include butare not limited to, chloromethyl, fluoromethyl, difluoromethyl,trifluoromethyl and 1-chloro-2-fluoromethyl, 2-fluoroisobutyl. Ahaloalkyl may be substituted or unsubstituted.

As used herein, a “substituted” group may be substituted with one ormore group(s) individually and independently selected from alkyl,alkenyl, alkynyl, cycloalkyl, cycloalkenyl, cycloalkynyl, benzyl,substituted benzyl, aryl, heteroaryl, heteroalicyclyl, aralkyl,heteroaralkyl, (heteroalicyclyl)alkyl, hydroxy, protected hydroxyl,alkoxy, aryloxy, acyl, mercapto, alkylthio, arylthio, cyano, halogen,thiol, thiocarbonyl, O-carbamyl, N-carbamyl, O-thiocarbamyl,N-thiocarbamyl, C-amido, N-amido, S-sulfonamido, N-sulfonamido,C-carboxy, protected C-carboxy, O-carboxy, isocyanato, thiocyanato,isothiocyanato, nitro, silyl, sulfenyl, sulfinyl, sulfonyl, haloalkyl,haloalkoxy, trihalomethanesulfonyl, trihalomethanesulfonamido, an amino,a mono-substituted amino group and a di-substituted amino group, andprotected derivatives thereof.

As used herein, “halogen” refers to an atom of fluorine, chlorine,bromine or iodine.

As used herein, “homocyclic ring” refers to cycloalkyl or aryl.

As used herein, “heterocyclic ring” refers to a ring, which may contain1 to 4 heteroatoms selected from among nitrogen, oxygen, sulfur andother atoms in addition to carbon atoms.

Exemplary cathinones or synthetic cathinones include, but are notlimited to, 3,4-methylenedioxypyrovalerone (MDPV);4′-methyl-α-pyrrolidinohexanophenone (MPHP); naphyrone; methylone;ethylone; butylone; pentylone; mephedrone; mexedrone; buphedrone;pentedrone; hexedrone; heptedrone; α-pyrrolidinopropiophenone (α-PPP);4′-methyl-α-pyrrolidinopropiophenone (M-α-PPP);3′,4′-methylenedioxy-α-pyrrolidinopropiophenone (MDPPP);1-phenyl-2-(1-pyrrolidinyl)-1-pentanone (α-PVP);α-pyrrolidinohexiophenone (α-PHP); α-pyrrolidinoheptiophenone (α-PHpP,PV8); diethylpropion; pyrovalerone; dimethylcathinone; diethylcathinone;methcathinone; ethcathinone; 3-methylmethcathinone (3-MMC);4-methylethcathinone (4-MEC); 3-chloromethcathinone (3-CMC);4-chloromethcathinone (4-CMC); n-ethyl-nor-pentedrone (NEP);3,4-methylenedioxy-α-pyrrolidinobutiophenone (MDPBP);4-methyl-α-pyrrolidinobutiophenone (MEPBP); 4-fluoromethcathinone(4-FMC); n-ethyl-nor-hexedrone (Hexen); n-ethyl-nor-heptedrone;4-ethylpentedrone; 4-methyl-NEP; and n-ethyl-nor-pentylone.

In a specific embodiment, the synthetic cathinone is selected from MDPV,penthylone, mephedrone, naphyrone, MDPBP, methylone, methedrone,ethylone, butylone, MPHP, and MEPBP.

Aptamer-Based Sensors

The subject invention provides aptamer-based sensors for rapid andnaked-eye detection of small-molecule targets. In one embodiment, theaptamer-based sensor is a CBSAzyme-based sensor having bothtarget-mediated cooperative behavior of the cooperative binding splitaptamers (CBSAs) and peroxidase-mimicking catalytic activity of DNAzyme.

In one embodiment, the CBSAzyme-based sensor comprises a CBSA-DNAzymeconjugate (CBSAzyme), which comprises or consists of a pair of highlytarget-responsive CBSAs grafted to an engineered split DNAzyme withperoxidase-mimicking catalytic activity. The CBSAzyme-based sensor canfurther comprise a signal reporter, which comprises a mixture of aperoxidase substrate and H₂O₂.

In one embodiment, the CBSAzyme comprises a split DNAzyme grafted to apair of CBSA fragments. In a further embodiment, the CBSAzyme comprisesa first fragment and a second fragment, the first fragment comprising afirst segment of a split DNAzyme and a long fragment of a CBSA; and thesecond fragment comprising a second segment of the split DNAzyme and ashort fragment of the CBSA.

The two fragments of the CBSAzyme remain separate in the absence of asmall-molecule target, but effectively assemble in the presence of thesmall-molecule target. The assembly of the two fragments of the CBSAzymeactivates the split DNAzyme that subsequently catalyzes the oxidation ofa peroxidase substrate in the presence of H₂O₂, producing a signalchange (e.g., a visible color change from colorless to dark green) thatreveals the presence and quantity of the target within minutes.

In one embodiment, the peroxidase substrate is selected from3,3′,5,5′-tetramethylbenzidine, ABTS,N-(4-Aminobutyl)-N-ethylisoluminol, 3-amino-9-ethylcarbazole,3-amino-9-ethylcarbazole, 4-aminophthalhydrazide, 5-aminosalicylic acid,2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid),4-chloro-1-naphthol, 4-chloro-7-nitrobenzofurazan, 3,3′-diaminobenzidinetetrahydrochloride, o-dianisidine dihydrochloride, iodonitrotetrazoliumchloride, luminol, 1-(4,5-dimethylthiazol-2-yl)-3,5-diphenylformazan,nitrotetrazolium blue chloride, o-phenylenediamine,trans-5-phenyl-4-pentenyl hydroperoxide, pyrogallol, tetranitrobluetetrazolium chloride, and tetrazolium Violet. In a preferred embodiment,the peroxidase substrate is ABTS.

In one embodiment, the CBSAzyme further comprises linkers between thefragments of the CBSA and the segments of DNAzyme, which include a firstlinker between the long fragment of the CBSA and the first segment ofthe split DNAzyme, and a second linker between the short fragment of theCBSA and the second segment of the split DNAzyme.

The linkers are nucleotide sequences comprising at least one, at leasttwo, at least three, at least four, at least five, at least six, atleast seven or at least eight nucleotides. In a preferred embodiment,the linkers comprise one or two nucleotides. The first and secondlinkers may have same or different sequences. In a specific embodiment,the linkers are selected from A, C, T, AA, AC, AT, CC, CA, CT, TA, TC,and TT.

In one embodiment, the CBSAzyme-based sensor further comprises acompound or cofactor that contributes to the activity of the splitDNAzyme. In a specific embodiment, the compound is hemin.

Aptamers

Aptamers are nucleic acid molecules characterized by the ability to bindto a target molecule with high specificity and high affinity. Almostevery aptamer identified to date is a non-naturally occurring molecule.Aptamers to a given target may be identified and/or produced by themethod of systematic evolution of ligands by exponential enrichment(SELEX). In one embodiment, the aptamer according to the subjectinvention is isolated by SELEX for the small molecule of interest.

In one embodiment, the aptamer is an oligonucleotide, such as DNA or RNAmolecules and may be single-stranded or double-stranded. In a preferredembodiment, the aptamer is a DNA aptamer.

The aptamer may be partially or fully folded to form various secondarystructures (e.g., stems, loops, bulges, pseudoknots, G-quadruplexes andkissing hairpins), which in turn can form unique three-dimensionalarchitectures able to specifically recognize their targets by exploitinga variety of interactions-such as hydrophobic and electrostaticinteractions, hydrogen bonding, van der Waals forces, and n-n stackingas well as shape complementarity.

As used herein, the terms “polynucleotide,” “nucleotide,”“oligonucleotide,” and “nucleic acid” refer to a nucleic acid comprisingDNA, RNA, derivatives thereof, or combinations thereof.

In certain embodiments, the aptamer according to the present inventionmay comprise at least 10, at least 20, at least 30, at least 40, atleast 50, at least 60, at least 70, or at least 80 nucleotides. Theaptamer according to the present invention, preferably, comprises 10 to200 nucleotides, preferably 15 to 150 nucleotides, more preferably 20 to100 nucleotides, most preferably, 30 to 60 nucleotides.

In certain embodiments, the aptamer according to the present inventionhas a minimum length of, for example, 10, 11, 12, 13, 14, 15, 16, 17,18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35,36, 37, 38, 39, or 40 nucleotides. The aptamer according to the presentinvention may have a maximum length of, for example, 50, 51, 52, 53, 54,55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72,73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90,91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106,107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120,121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134,135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148,149, or 150 nucleotides. The aptamer according to the present inventionmay have a length of, for example, 10, 11, 12, 13, 14, 15, 16, 17, 18,19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36,37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54,55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72,73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90,91, 92, 93, 94, 95, 96, 97, 98, 99, or 100 nucleotides.

In some embodiments, the aptamers according to the subject inventionhave free ends. For example, the 3′ and 5′ ends may not be ligated toform a loop, although they may be conjugated to other molecules orotherwise modified. The aptamers may adopt a tertiary structure such asa hairpin loop.

In certain embodiments, the aptamer according to the subject inventioncomprises at least one stems, two stems, or three stems. Preferably, theaptamer comprises three stems. Each stem may be fully or partiallycomplementary. Each stem may comprise the same or a different number ofnucleotides. Exemplary lengths of each stem may be 4, 5, 6, 7, 8, 9, 10,11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28,29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, or 40 nucleotides.

In one embodiment, each stem comprises the same or a different number ofbase pairs (bps). Each stem may have a minimum of, for example, 1, 2, 3,4, 5, 6, 7, 8, 9, 10, 11, or 12 base pairs. Each stem may have a maximumof, for example, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26,27, 28, 29, or 30 base pairs. Each stem according to the presentinvention may have a number of, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9,10, 11, 12, 13, 14, or 15 base pairs. A partially complementary stem maycomprise more than one wobble base pair, including, but not limited to,G-U, and T-G.

Each of the stems may independently connect to a loop at the end,forming a stem-loop structure. The aptamer may thereby comprise at leastone, two, or three stem-loop structures. The stem-loop structureaccording to the present invention may have a minimum length of, forexample, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21,22, 23, 24, or 25 nucleotides. The stem-loop structure may have amaximum length of, for example, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49,50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, or 65nucleotides.

In one embodiment, the aptamer comprises at least one junction, which isformed when two or more stems meet. In certain embodiments, the junctionmay be a loop between two stems, or a three-way junction (TWJ). Thejunction in an aptamer can serve as a binding domain for asmall-molecule target.

The junction may have a minimum length of, for example, 3, 4, 5, 6, 7,8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25,26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, or 40nucleotides. The junction may have a maximum length of, for example, 41,42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59,60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77,78, 79, or 80 nucleotides. The junction may comprise, for example, 5, 6,7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25,26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43,44, 45, 46, 47, 48, 49, or 50 nucleotides.

In a specific embodiment, the aptamer comprises a TWJ-binding domain. Insome embodiments, the aptamer is a monomer, dimer, trimer, or tetramer.Such aptamer can comprise one, two, three, or four TWJ-binding domains.The aptamer containing one or more TWJ-binding domain may bepredominantly folded even in the absence of the target due to themultiple Watson-Crick base pairs in its stem. Such aptamer can bind totheir target with micro or nano-molar affinity.

CBSA

CBSAs comprise a pair of split aptamer fragments (i.e., a short fragmentand a long fragment) with two tandem target-binding domains, whichexhibit cooperative binding properties. CBSAs can be engineered fromparent split aptamers comprising a single target-binding domain.Advantageously, CBSAs with two target-binding domains exhibit enhancedtarget response compared with single-domain split aptamers. The firstbinding event partially stabilizes the CBSA and facilitates the secondbinding event. This cooperative assembly can reduce the targetconcentration required to assemble the CBSA several-fold. Thus, CBSAsare highly target-responsive and have superior sensitivity compared totheir single-site split aptamer counterparts.

In one embodiment, each piece of the split aptamer according to thepresent invention may have a minimum length of, for example, 5, 6, 7, 8,9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26,27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, or 40 nucleotides.Each piece of the split aptamer according to the present invention mayhave a maximum length of, for example, 40, 41, 42, 43, 44, 45, 46, 47,48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65,66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, or 80nucleotides. Each piece of the split aptamer according to the presentinvention may have a length of, for example, 5, 6, 7, 8, 9, 10, 11, 12,13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30,31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48,49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66,67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, or 80 nucleotides.

In one embodiment, each fragment of CBSA according to the presentinvention may have a minimum length of, for example, 10, 11, 12, 13, 14,15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32,33, 34, 35, 36, 37, 38, 39, or 40 nucleotides. Each fragment of CBSAaccording to the present invention may have a maximum length of, forexample, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55,56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73,74, 75, 76, 77, 78, 79, or 80 nucleotides. Each fragment of CBSAaccording to the present invention may have a length of, for example,10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27,28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45,46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63,64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, or 80nucleotides.

The CBSAs according to the present invention may comprise at least 10,at least 20, at least 30, at least 40, at least 50, at least 60, atleast 70, or at least 80 nucleotides. The CBSAs according to the presentinvention may comprise at most about 200 nucleotides, at most about 150nucleotides, at most about 120 nucleotides, at most about 100nucleotides, at most about 90 nucleotides, at most about 80 nucleotides,at most about 70 nucleotides, at most about 60 nucleotides, or at mostabout 50 nucleotides. The aptamer according to the present inventioncomprises, for example, in the range of 10 to 200 nucleotides,preferably 15 to 150 nucleotides, more preferably 20 to 100 nucleotides.

In one embodiment, the CBSA is a cocaine-binding CBSA, includingCOC-5335 (LF: SEQ ID NO: 8; SF: SEQ ID NO: 9), COC-5334 (LF: SEQ ID NO:17; SF: SEQ ID NO: 16) and COC-5333 (LF: SEQ ID NO: 34; SF: SEQ ID NO:35) and sequences sharing at least 50%, 55%, 60%, 65%, 70%, 75%, 80%,85%, 90%, 95% or 99% identity with COC-5335, COC-5334 and COC-5333.

In another embodiment, the CBSA is a synthetic cathinone-binding CBSA.In a specific embodiment, the CBSA is a MDPV-binding CBSA, MDPV-6335(LF: SEQ ID NO: 25; SF: SEQ ID NO: 26) and sequences sharing at least50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 99% identity withMDPV-6335.

The MDPV-6335 is engineered from an MDPV-binding aptamer (SEQ ID NO: 24)comprising a TWJ binding domain and three stems. The MDPV-bindingaptamer binds to MDPV with a K_(D) of 6 μM. Two different parent splitaptamer pairs are derived with a single binding pocket from theMDPV-binding aptamer, in which the GAA loop from stem 3 is removed andthe number of base-pairs in all stems is decreased. Stem 1 of one parentsplit aptamer is then connected to stem 3 of the second parent splitaptamer via a single thymine linker on each strand to form aMDPV-binding CBSA, MDPV-6335, comprising a long fragment and a shortfragment.

In the absence of MDPV, the two fragments remain separate. When thetarget is added, the CBSA assembles to form a rigid target-CBSA complex.MDPV-6335 has a binding affinity (K_(1/2), target concentrationproducing half occupancy) of 140.6 μM with a cooperativity (n_(H)) of1.8, which shows the high degree of target binding cooperativity.

The aptamers/split aptamers/CBSAs of the present invention may includechemical modifications. The chemical modifications as described hereininclude a chemical substitution at a sugar position, a phosphateposition, and/or a base position of the nucleic acid including, forexample, incorporation of a modified nucleotide, incorporation of acapping moiety (e.g., 3′ capping), conjugation to a high molecularweight, non-immunogenic compound (e.g., polyethylene glycol (PEG)),conjugation to a lipophilic compound, and substitutions in the phosphatebackbone. Base modifications may include 5-position pyrimidinemodifications, modifications at exocyclic amines, substitution of4-thiouridine, substitution of 5-bromo- or 5-iodo-uracil, and backbonemodifications. Sugar modifications may include 2′-amine nucleotides(2′-NH2). 2′-fluoronucleotides (2′-F), and 2′-O-methyl (2′-OMe)nucleotides. Such modifications may improve the stability of theaptamers or make the aptamers more resistant to degradation. In someembodiments, each base of a given type (e.g., A, T, C, and G) maycontain the same chemical modification.

The aptamers/split aptamers/CBSAs may be modified by addition of one ormore reporter labels (or detectable labels). In some embodiments, thelabel may be attached to either the 5′ or 3′ end of the aptamer. Thelabel may also be attached with the backbone of the aptamer. The skilledperson will be aware of techniques for attaching labels to nucleic acidstrands. The detectable label may be attached directly or indirectly tothe nucleic acid aptamer. If the label is indirectly attached to thenucleic acid aptamer, it may be by any mechanism known to one of skillin the art, such as using biotin and streptavidin.

The aptamers/split aptamers/CBSAs may comprise a reporter label, such asa fluorescent dye, nanoparticle, or an enzyme. Exemplary labels include,but are not limited to, an organic donor fluorophore or an organicacceptor fluorophore, a luminescent lanthanide, a fluorescent orluminescent nanoparticle, an affinity tag such as biotin, or apolypeptide. In some embodiments, the aptamer may comprise a fluorescentlabel, for example, fluorescein, TAMRA, rhodamine, Texas Red, AlexaFluor (e.g., AlexaFluor 488, AlexaFluor 532, AlexaFluor 546, AlexaFluor594, AlexaFluor 633 and AlexaFluor 647), cyanine dye (e.g., Cy7, Cy7.5,Cy5, Cy5.5 and Cy3), Tye dye (e.g., TYE 563, TYE 665, TYE 705), atto dye(e.g., Atto 594 and Atto 633), Hexachlorofluorescein, FAM(6-carboxyfluroescein), BODIPY FL, OliGreen,40,6-diamidino-2-phenylindol (DAPI), Hoechst 33,258, malachite green(MG), and FITC. The nanoparticle can be an upconversion nanoparticle. Insome embodiments, the fluorophore is selected from the group consistingof fluorophores that emit a blue, green, near red or far redfluorescence.

In one embodiment, the reporter label is a fluorescent dye and quencherpair. In certain embodiments, a fluorophore is conjugated at one end ofthe short fragment of CBSAs and a quencher at the other end of the shortfragment of CBSAs. In the absence of its target, the short fragment ofthe CBSA is flexible, thereby positioning the fluorophore close to thequencher, and the fluorescence is quenched. Upon target binding, CBSAfragments assemble into a rigid conformation, in which the fluorophoreand the quencher are separated and the fluorescence is recovered. Theresulting recovery of the fluorescence signal directly reflects theextent of the binding and can be used for detection and quantitativemeasurement of the target concentration.

The quenchers can be, for example, Dabcyl, DDQ-1, Eclipse, Iowa BlackFQ, BHQ-1, QSY-7, BHQ-2, DDQ-II, Iowa Black RQ, QSY-21, or BHQ-3.

It is contemplated that the location of the fluorophore andquencher-conjugated short fragment is such that the proximity offluorophore and quencher provide maximal quenching in an single-strandedflexible conformation and the fluorophore and quencher in an assembledrigid conformation provide maximal fluorescence of the fluorophore. Foroptimized detection of fluorescence changes that allows utilization ofthe CBSAs for target detection, it is desirable that the fluorescence inthe quenched conformation is as low as possible and the fluorescence inthe unquenched conformation is as high as possible combined with themost rapid interconversion from one conformation to the other.

In one embodiment, the CBSA-target complex may produce a time resolvedfluorescence energy transfer (TR-FRET) signal or a signal that can bemeasured by fluorescence polarization (FP), and/or luminescence.

In one embodiment, the aptamer binds to the small-molecule target with adissociation constant of, for example, about 1 μM, about 2 μM, about 3μM, about 4 μM, about 5 μM, about 6 μM, about 7 μM, about 8 μM, about 9μM, or about 10 μM. In specific examples, The aptamer binds to the smallmolecule with a dissociation constant between about 0.001 μM and about1000 μM, between about 0.01 μM and about 500 μM, between about 0.1 μMand about 200 μM, between about 0.5 μM and about 100 μM, between about 1μM and about 100 μM, between about 1 μM and about 50 μM, between about 1μM and about 30 μM, between about 1 μM and 20 μM, or between about 1 μMand about 10 μM.

DNAzyme

The DNAzymes are G-quadruplex-structured oligonucleotides that mimic theactivity of horseradish peroxidase, and are capable of binding hemin andperforming H₂O₂-mediated oxidation of colorless small-moleculesubstrates into colored products. DNAzymes have several desirablecharacteristics, including their signal-amplifying ability, chemicalstability, ease of mass production without any batch-to-batch variation,and the simplicity of incorporating them with different sensingelements.

In one embodiment, the DNAzyme may comprise, for example, at least 1, 2,3, 4, 5, 6, 7, 8, 9, or 10 GGG repeats. The DNAzyme can be split intotwo fragments, either in a symmetrical or asymmetrical fashion. As aresult, each segment of the split DNAzyme may comprise a same ordifferent number of GGG repeats. For example, a DNAzyme comprising 4 GGGrepeats can be symmetrically split into two segments, each of whichcomprises 2 GGG repeats, resulting in a 2:2 split. Other exemplary splitmodes include, for example, 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 2:2,2:3, 2:4, 2:5, 2:6, 2:7, 2:8, 3:3, 3:4, 3:5, 3:6, 3:7, 4:4, 4:5, 4:6,4:3, 4:2, 4:1, 5:5, 5:4, 5:3, 5:2, 5:1, 6:4, 6:3, 6:2, 6:1, 7:3, 7:2,7:1, 8:2, 8:1, and 9:1.

In one embodiment, the split DNAzyme segments can be reconstituted withhemin when they are brought into close proximity to form a layeredG-quadruplex complex that exhibits peroxidase-like activity, catalyzingthe oxidation of colorless ABTS into dark green ABTS^(•+) by H₂O₂.

In one embodiment, the split DNAzyme further comprises at least onespacer between the guanine triplets or within the GGG repeats. In someembodiments, there may be spacers between every guanine triplet. In afurther embodiment, one fragment of the split DNAzyme comprises asequence of 5′-GXGGYGGG-3′ (SEQ ID NO: 36), wherein X and Y are spacers,and X and Y may be missing. The other fragment of the split DNAzymecomprises a sequence of 5′-GGGZGGG-3′ (SEQ ID NO: 37), wherein Z is aspacer and Z may be missing.

In one embodiment, the spacer is a nucleotide sequence comprising atleast one, at least two, at least three, at least four, at least five,at least six, at least seven or at least eight nucleotides. In apreferred embodiment, the spacer comprises one or two nucleotides. Thespacers may be selected from A, C, T, AA, AC, AT, CC, CA, CT, TA, TC,and TT. Preferably, the spacer provides better flexibility for DNAzymeassembly, thereby resulting in higher catalytic activity. A shorterspacer might be preferred due to the formation of a more compactG-quadruplex structure that boosts catalytic activity.

In a specific embodiment, the catalytic activity of the 2:2 splitDNAzyme is improved by two-fold via rational mutation and deletion ofnucleotides within and between the GGG repeats.

In some embodiments, the spacer X, Y, and Z each independently comprisesone or two nucleotides. Preferably, the spacer X is T, Y is A or C, andZ is AC or C.

In specific embodiments, the split DNAzyme comprises a fragment pairfrom strands A0 and B0, A1 and B0, A2 and B0, or A2 and B1, wherein thefragment from A0 comprises 5′-GTGGAGGGT-3′(SEQ ID NO: 38), the fragmentfrom A1 comprises 5′-GGGAGGGT-3′ (SEQ ID NO: 39), the fragment from A2comprises 5′-GGGCGGGT-3′ (SEQ ID NO: 40), the fragment from B0 comprises5′-AGGGACGGG-3′ (SEQ ID NO: 41), and the fragment from B1 comprises5′-AGGGCGGG-3′ (SEQ ID NO: 42).

CBSAzyme

In one embodiment, the subject invention provides a CBSAzyme-basedsensor, comprising a pair of split DNAzyme segments and a pair of CBSAsfragments selected to a small-molecule target. In one embodiment, theCBSAzyme-based sensor comprises a pair of highly target-responsive CBSAsand an engineered split DNAzyme with peroxidase-mimicking catalyticactivity, wherein the pair of CBSA fragments is coupled to the splitDNAzyme segments. The CBSAzyme-based sensor further comprises a signalreporter, which comprises a mixture of a peroxidase substrate and H₂O₂

In one embodiment, the CBSAzyme comprises or consists of twooligonucleotide fragments, each with a CBSA region containing two tandemtarget-binding domains and a split horseradish-peroxidase-mimickingDNAzyme module. CBSAzymes are ideal sensing elements for small-moleculedetection as they inherit the high target responsiveness of CBSAs andthe catalytic activity of DNAzymes, which enables label-free, amplifiednaked-eye detection.

In one embodiment, the CBSAzyme comprises a first fragment (i.e., longfragment) and a second fragment (i.e., short fragment), the firstfragment comprising a first segment of the split DNAzyme and a longfragment of the CBSA, the second fragment comprising a second segment ofthe split DNAzyme and a short fragment of the CBSA.

In one embodiment, the pair of CBSA fragments is coupled to the pair ofsplit DNAzyme segments via nucleotide linkers. The linkers locatebetween the fragments of the CBSA and the segments of DNAzyme. In afurther embodiment, the CBSAzyme comprises a first linker between thelong fragment of the CBSA and the first segment of the split DNAzyme,and a second linker between the short fragment of the CBSA and thesecond segment of the split DNAzyme.

The linkers are nucleotide sequences comprising at least one, at leasttwo, at least three, at least four, at least five, at least six, atleast seven or at least eight nucleotides. In a preferred embodiment,the linkers comprise one or two nucleotides. The first and secondlinkers may have same or different sequences. In a specific embodiment,the linkers are selected from A, C, T, AA, AC, AT, CC, CA, CT, TA, TC,and TT.

In the absence of a small-molecule target, the two fragments of theCBSAzyme remain separate. In the present of the small-molecule target,the two fragments of the CBSA in CBSAzyme effectively assemble, bringingthe two DNAzyme segments into close proximity. When hemin is present,the assembled DNAzyme module accommodates hemin within its G-quadruplex,allowing for H₂O₂-mediated oxidation of ABTS to ABTS^(•+), therebyrapidly turning the solution from colorless to dark green.

In one embodiment, the CBSAzyme is a selective for cocaine, comprising aCBSA selective for cocaine and a pair of split DNAzyme segments. In aspecific embodiment, the CBSA is selected from i) COC-5335 having a longfragment of SEQ ID NO: 8 and a short fragment of SEQ ID NO: 9; ii)COC-5334 having a long fragment of SEQ ID NO: 17 and a short fragment ofSEQ ID NO: 16; and iii) COC-5333 a long fragment of SEQ ID NO: 34 and ashort fragment of SEQ ID NO: 35. The split DNAzyme comprises segmentsfrom strands A2 and B0.

In a specific embodiment, the CBSAzyme is CBSAzyme-5335-22, where theDNAzyme segment from Strand A2 (5′-GGGCGGGT-3′) is attached to the 3′terminus of the long fragment of COC-5335 via an A-T dinucleotidelinker, and the DNAzyme segment from strand B1 (5′-AGGGCGGG-3′) isattached to the 5′ terminus of the short fragment of COC-5335 via an A-Adinucleotide linker. In another embodiment, the DNAzyme segment fromstrand A2 (5′-GGGCGGGT-3′) is attached to the 3′ terminus of the longfragment of COC-5335 via an A linker. Preferably, CBSAzyme-5335-22 has along fragment having a sequence of SEQ ID NO: 10 and a short fragmenthaving a sequence of SEQ ID NO: 11.

In the absence of cocaine, these two CBSAzyme fragments are separatedand the split DNAzyme remains unassembled and incapable of oxidizingABTS. Thus, the solution remains clear. In the presence of cocaine, bothfragments assemble, bringing the two DNAzyme segments into closeproximity. The assembled DNAzyme module accommodates hemin within itsG-quadruplex, allowing for H₂O₂-mediated oxidation of ABTS to ABTS^(•+),producing a dark green-colored solution, which can be seen by nakedeyes.

In one embodiment, the CBSAzyme is CBSAzyme-5334-22, comprising a pairof CBSA fragments, COC-5334, in which one base pair from the termini ofthe CBSA fragments of COC-5335 closest to the DNAzyme segments istruncated. The DNAzyme segment from strand A2 (5′-GGGCGGGT-3′) isattached to the 3′ terminus of the long fragment of COC-5334 via an A-Tdinucleotide linker, and the DNAzyme segment from strand B1(5′-AGGGCGGG-3′) is attached to the 5′ terminus of the short fragment ofCOC-5334 via an A-A dinucleotide linker. In another embodiment, theDNAzyme segment from strand A2 (5′-GGGCGGGT-3′) is attached to the 3′terminus of the long fragment of COC-5334 via an A linker. Preferably,CBSAzyme-5334-22 has a long fragment having a sequence of SEQ ID NO: 12and a short fragment having a sequence of SEQ ID NO: 13.

In one embodiment, the CBSAzyme is CBSAzyme-5333-22, comprising a pairof CBSA fragments, COC-5333, in which two base pairs from the termini ofthe CBSA fragments of COC-5335 closest to the DNAzyme segments aretruncated. The DNAzyme segment from strand A2 (5′-GGGCGGGT-3′) isattached to the 3′ terminus of the long fragment of COC-5333 via an A-Tdinucleotide linker, and the DNAzyme segment from strand B1(5′-AGGGCGGG-3′) is attached to the 5′ terminus of the short fragment ofCOC-5333 via an A-A dinucleotide linker. In another embodiment, theDNAzyme segment from strand A2 (5′-GGGCGGGT-3′) is attached to the 3′terminus of the long fragment of COC-5333 via an A linker. Preferably,CBSAzyme-5333-22 has a long fragment having a sequence of SEQ ID NO: 14and a short fragment having a sequence of SEQ ID NO: 15.

In one embodiment, the CBSAzyme is CBSAzyme-5334-13, comprisingCOC-5334, and a split DNAzyme having a 1:3 split mode, in which onesegment of the split DNAzyme comprises 1 GGG repeat while the othersegment of the split DNAzyme comprises 3 GGG repeats. In a furtherembodiment, the DNAzyme segment having three GGG repeats is coupled tothe short fragment of the CBSA (COC-5334-SF) via an A-A dinucleotidelinker. The DNAzyme segment having 1 GGG repeat is coupled to the longfragment of the CBSA (COC-5334-LF) via an A-T dinucleotide linker or anA linker. In a specific embodiment, the DNAzyme segment having three GGGrepeats comprises two spacers between the GGG repeats. In a preferredembodiment, the spacers are C and AT.

Advantageously, CBSAzyme-5334-13 demonstrates much lower backgroundassembly relative to CBSAzyme-5334-22 in the absence of cocaine, mostlikely due to the low stability of 1:3 split DNAzymes. The 1:3 splitCBSAzyme also has a higher level of activity in the presence of cocainerelative to the 2:2 split CBSAzyme.

In a preferred embodiment, the CBSAzyme is COC-CBSAzyme (SF: SEQ ID NO:19 and LF: SEQ ID NO: 20) comprising the A/AA linker combination whichfavors assembly of both the CBSA and the split DNAzyme.

In some embodiments, the aptamer is specific for cocaine and does notbind to various structurally similar drugs such as benzoylecgonine(BZE), and methylecgonidine (MEG) as well as structurally dissimilarinterferent drugs such as nicotine (NIC), and scopolamine (SCP).

In one embodiment, the CBSAzyme is a MDPV-CBSAzyme, comprising a CBSAthat binds to MDPV and other synthetic cathinones, and a split DNAzyme.In a specific embodiment, the CBSA is MDPV-6335. The split DNAzymehaving a 1:3 split mode, in which one segment of the split DNAzymecomprises 1 GGG repeat while the other segment of the split DNAzymecomprises 3 GGG repeats. In a further embodiment, the DNAzyme segmenthaving three GGG repeats is coupled to the short fragment of MDPV-6335(MDPV-6335-SF) via an A-A dinucleotide linker. The DNAzyme segmenthaving 1 GGG repeat is coupled to the long fragment of MDPV-6335(MDPV-6335-LF) via an A linker. In a specific embodiment, the DNAzymesegment having three GGG repeats comprises two spacers between the GGGrepeats. In a preferred embodiment, the spacers are C and AT.

In some embodiment, the split DNAzyme comprises segments from strandsA0, A1, A2, B1 and B0. For example, the DNAzyme segment from Strand A2(5′-GGGCGGGT-3′) may be attached to the 3′ terminus of MDPV-6335-LF viaan A-T dinucleotide linker or an A linker, and the DNAzyme segment fromStrand B1 (5′-AGGGCGGG-3′) maybe attached to the 5′ terminus ofMDPV-6335-SF via an A-A dinucleotide linker.

Preferable, the MDPV-CBSAzyme has a long fragment having a sequence ofSEQ ID NO: 28 or a sequence sharing at least 50%, 55%, 60%, 65%, 70%,75%, 80%, 85%, 90%, 95% or 99% identity with SEQ ID NO: 28, and a shortfragment having a sequence of SEQ ID NO: 29 or a sequence sharing atleast 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 99% identitywith SEQ ID NO: 29.

Advantageously, the MDPV-CBSAzyme retains excellent cross-reactivity tostructurally-similar synthetic cathinone analogs including MDPV,methylone, pentylone, 3,4-methylenedioxy-α-pyrrolidinobutiophenone(MDPBP), mephedrone, 4-methyl-α-pyrrolidinobutiophenone (MPBP),4′-methyl-α-pyrrolidinohexanophenone (MPHP), naphyrone, methedrone,ethylone, and butylone but does not bind to interferents, includingcommon cutting agents and illicit drugs such as caffeine, benzocaine,lidocaine, sucrose and methamphetamine.

Importantly, the stem length of the CBSA is crucial for achieving highsignal gain and low background assembly. The split mode of the DNAzymehas a profound impact on CBSAzyme performance, for example, changing the2:2 split mode to 1:3 results in higher enzymatic activity in thepresence of target and lower background signal when the target isabsent. The length of the linker also had an effect on signal gain, inwhich mononucleotide and dinucleotide linkers are optimal for the longand short CBSAzyme fragments, respectively. This particular combinationof linkers provides flexibility for DNAzyme assembly with minimal sterichindrance.

In one embodiment, the CBSAzyme, according to the subject invention, isselected from i) COC-CBSAzyme-5335-22 having a first fragment of SEQ IDNO: 10 and a second fragment of SEQ ID NO: 11; ii) COC-CBSAzyme-5334-22having a first fragment of SEQ ID NO: 12 and a second fragment of SEQ IDNO: 13; iii)) COC-CBSAzyme-5333-22 having a first fragment of SEQ ID NO:14 and a second fragment of SEQ ID NO: 15; iv) COC-CBSAzyme-5334-13(AT/AA) having a first fragment of SEQ ID NO: 18 and a second fragmentof SEQ ID NO: 19; v) COC-CBSAzyme-5334-13 (A/AA) having a first fragmentof SEQ ID NO: 20 and a second fragment of SEQ ID NO: 19; vi)COC-CBSAzyme-5334-13 (A/A) having a first fragment of SEQ ID NO: 20 anda second fragment of SEQ ID NO: 21; and vii) MDPV-CBSAzyme having afirst fragment of SEQ ID NO: 28 and a second fragment of SEQ ID NO: 29.

Method of Using CBSAzyme

The subject invention provides methods of using the CBSAzyme-basedassays for rapid and naked-eye detection of small-molecule targets in asample. The method comprises contacting the sample with a CBSAzyme-basedsensor selective for the small-molecule target, and detecting thesmall-molecule target in the sample.

Advantageously, the method utilizes CBSAzymes to achieve sensitive,naked-eye small-molecule detection, a feat that cannot be achieved withany other split-aptamer-DNAzyme conjugates. Because CBSAzymes areentirely based on nucleic acids, with no chemical modificationsrequired, they can be easily synthesized in a cost-effective andreproducible manner. Additionally, the CBSAzymes themselves arechemically stable and resistant to harsh environmental conditions.Moreover, CBSAzyme-based assays require no instrumentation, and aresimple to perform-requiring only sample-reagent mixing and visualinterpretation.

The CBSAzyme-based sensor comprises a CBSAzyme comprising a pair of CBSAfragments and a split DNAzyme with peroxidase-mimicking catalyticactivity. The CBSAzyme comprises two fragments, one of which comprises along fragment of the CBSA linked to one segment of the split DNAzyme andthe other of which comprises a short fragment of the CBSA linked toanother segment of the split DNAzyme. The CBSAzyme-based sensor furthercomprises a signal reporter, which comprises a mixture of a peroxidasesubstrate and H₂O₂.

In the absence of the small-molecule target, the two fragments of theCBSAzyme remain separate and thus the unassembled split DNAzyme moduleremains inactive and incapable of oxidizing the peroxidase substrate(e.g., ABTS). In the present of the small-molecule target, the twofragments of the CBSA in CBSAzyme effectively assemble, bringing the twoDNAzyme segments into close proximity. When hemin is present, theassembled DNAzyme forms the G-quadruplex structure which accommodateshemin, allowing for H₂O₂-mediated oxidation of the peroxidase substrate(e.g., from ABTS to ABTS^(•+)), thereby rapidly changing the color ofthe reaction solution (e.g., turning the solution from colorless to darkgreen). Advantageously, this method provides very low levels ofbackground signal because the fragments only underwent minimal assemblyin the absence of target.

The method of the subject invention is remarkably simple, fast andspecific. For example, the detection can be performed in a single tubecontaining the CBSAzyme-based sensor and the sample of interest.

In one embodiment, the subject invention provides a method for rapid,sensitive and visual detection of a small-molecule target in a samplecomprising contacting the sample with a CBSAzyme-based sensor comprisinga CBSAzyme selective for the small-molecule target, and a signalreporter, the CBSAzyme comprising a pair of CBSA fragments grafted to apair of split DNAzyme segments, and detecting the small-molecule targetin the sample by determining whether a signal occurs upon thesmall-molecule target binding to the CBSAzyme, the signal being a colorchange, which is indicative of the presence of the small-molecule targetin the sample. The method may further comprise a step of contacting theCBSAzyme-based sensor with a compound that acts as a cofactor of thesplit DNAzyme.

In certain embodiments, the method for rapid, sensitive and visualdetection of a small-molecule target in a sample comprising

contacting the sample with a CBSAzyme selective for the small-moleculetarget, the CBSAzyme comprising a pair of CBSA fragments grafted to apair of split DNAzyme segments,

contacting the sample and CBSAzyme with a compound that acts as acofactor of the split DNAzyme,

adding a signal reporter, and

detecting the small molecule in the sample, the detection of thesmall-molecule target comprising measuring a signal generated from thesignal reporter.

In one embodiment, the detection of the small-molecule target comprisesmeasuring a signal generated upon contacting the sample with theCBSAzyme-based sensor. The signal generated upon contacting the samplewith the CBSAzyme-based sensor has optical properties that can bedetected by naked eyes and quantified by, for example, the absorption.

In another embodiment, the optical properties can be quantified by, forexample, a microplate-reader or portable photometer, allowing forhigh-throughput or on-site detection, respectively.

In one embodiment, the compound/cofactor for promoting the assembly ofthe split DNAzyme segments is hemin. The signal reporter comprises amixture of a peroxidase substrate (e.g., ABTS) and H₂O₂. The signalgenerated from the signal reporter is the color change resulted from theH₂O₂-mediated oxidation of the peroxidase substrate (e.g., from ABTS toABTS^(•+)) by the peroxidase-mimicking catalytic activity of the splitDNAzyme. Thus, a change in color is indicative of the presence of thesmall-molecule target in the sample. Such change in color can bedetected by naked eyes.

In another embodiment, the method further comprises determining theconcentration of the small-molecule target in the sample. Thedetermination can comprise comparing the absorption signal generatedupon contacting the sample with a CBSAzyme-based sensor with a standardcalibration curve of the absorption signal of the ABTS^(•+), forexample, at 418 nm in the presence of various concentrations of thesmall-molecule target.

In one embodiment, the subject invention provides a method for detectingcocaine in a sample wherein said method comprises contacting said samplewith a CBSAzyme-based sensor selective for cocaine and detecting cocainein the sample by determining whether a signal occurs upon cocainebinding to the CBSAzyme, the signal being a color change, which isindicative of the presence of cocaine in the sample. The CBSAzyme-basedsensor comprises a CBSAzyme selective for cocaine and a signal reporterbeing a mixture of a peroxidase substrate and H₂O₂, the peroxidasesubstrate being ABTS.

Because the color intensity of the solution was proportional to theconcentration of cocaine, cocaine concentrations as low as 10 μM can bedetected by naked eye within 5 minutes. A calibration curve using theabsorbance ABTS^(•+) at 418 nm after 15 minutes of reaction can be usedfor determining the concentration of cocaine and a detection limit of 1μM cocaine with a linear range from 0 to 100 μM is observed. The methodcan achieve successful cocaine detection in a biological sampleincluding body fluids such as saliva and urine.

In specific embodiments, the CBSAzyme-based sensor comprises a CBSAzymeselected from i) COC-CBSAzyme-5335-22 having a first fragment of SEQ IDNO: 10 and a second fragment of SEQ ID NO: 11; ii) COC-CBSAzyme-5334-22having a first fragment of SEQ ID NO: 12 and a second fragment of SEQ IDNO: 13; iii)) COC-CBSAzyme-5333-22 having a first fragment of SEQ ID NO:14 and a second fragment of SEQ ID NO: 15; iv) COC-CBSAzyme-5334-13(AT/AA) having a first fragment of SEQ ID NO: 18 and a second fragmentof SEQ ID NO: 19; v) COC-CBSAzyme-5334-13 (A/AA) having a first fragmentof SEQ ID NO: 20 and a second fragment of SEQ ID NO: 19; vi)COC-CBSAzyme-5334-13 (A/A) having a first fragment of SEQ ID NO: 20 anda second fragment of SEQ ID NO: 21. Preferably, the CBSAzyme isCOC-CBSAzyme-5334-13 (A/AA) having a first fragment of SEQ ID NO: 20 anda second fragment of SEQ ID NO: 19.

Advantageously, the method according to the subject invention can beused to specifically detect cocaine and does not respond to otherinterferent drugs because the CBSAzyme does not cross-react toscopolamine and sucrose and shows only minimal cross-reactivity forcaffeine, chlorpromazine, promazine, and levamisole, and moderatecross-reactivity for diphenhydramine and lidocaine.

In one embodiment, the subject invention provides a method for detectinga synthetic cathinone in a sample wherein said method comprisescontacting said sample with a CBSAzyme-based sensor comprising aCBSAzyme selective for the synthetic cathinone and determining whether achange in color occurs, wherein a change in color is indicative of thepresence of synthetic cathinone in the sample. Such change in color canbe detected by naked eyes.

In a specific embodiment, the CBSAzyme-based sensor comprises a CBSAzymebeing MDPV-CBSAzyme having a first fragment of SEQ ID NO: 28 and asecond fragment of SEQ ID NO: 29.

In a further embodiment, the subject invention provides a method fordetecting MDPV in a sample wherein said method comprises contacting saidsample with the CBSAzyme-based sensor that binds MDPV and determiningwhether a change in color occurs, wherein a change in color isindicative of the presence of MDPV in the sample.

Because the color intensity of the solution was proportional to theconcentration of MDPV, MDPV concentrations as low as 30 μM can bedetected by naked eye within 5 minutes. A calibration curve using theabsorbance ABTS^(•+) at 418 nm after 15 minutes of reaction can be usedfor determining the concentration of MDPV and a detection limit of 3 μMMDPV with a linear range from 0 to 100 μM is observed.

In one embodiment, the method according to the subject invention allowthe visual detection of a variety of synthetic cathinones including,MDPV, methylone, pentylone, 3,4-methylenedioxy-α-pyrrolidinobutiophenone(MDPBP), mephedrone, 4-methyl-α-pyrrolidinobutiophenone (MPBP),4′-methyl-α-pyrrolidinohexanophenone (MPHP), naphyrone, methedrone,ethylone, and butylone.

Advantageously, the CBSAzyme-based assay retained excellent specificityagainst interferents, including common cutting agents and illicit drugssuch as caffeine, benzocaine, lidocaine, sucrose and methamphetamine.

In one embodiment, the subject invention provide a method for visuallydetecting one or more synthetic cathinones in a sample, the methodcomprising contacting the sample with a CBSAzyme-based sensor comprisinga signal reporter and a CBSAzyme that binds to one or more syntheticcathinones, and detecting one or more synthetic cathinones in the sampleby determining whether a signal occurs upon one or more syntheticcathinones binding to the CBSAzyme, the signal being a color change,which is indicative of the presence of one or more synthetic cathinonesin the sample.

In one embodiment, the method according to the subject invention canachieve superior sensitivity for target detection at low micromolar ornanomolar concentration, for example, as low as about 200 μM, about 150μM, about 100 μM, about 10 μM, about 1 μM.

In one embodiment, the methods for small molecule detection providedherein are rapid and can be completed in about 5 minutes to about 120minutes, about 6 minutes to about 110 minutes, about 7 minutes to about100 minutes, about 8 minutes to about 90 minutes, about 9 minutes toabout 80 minutes, about 10 minutes to about 70 minutes about 15 minutesto about 60 minutes, about 20 minutes to about 50 minutes, or about 25minutes to about 40 minutes. In some embodiments, the method can becompleted in about 5 minutes, about 10 minutes, about 15 minutes, about20 minutes, about 25 minutes, about 30 minutes, about 35 minutes, about40 minutes, about 45 minutes, about 50 minutes, about 55 minutes, about60 minutes, about 65 minutes, about 70 minutes, about 75 minutes, about80 minutes, about 85 minutes, about 90 minutes, about 95 minutes, about100 minutes, about 105 minutes, about 110 minutes, about 115 minutes, orabout 120 minutes.

In another embodiment, the methods for small molecule detection providedherein are rapid and can be completed in about 5 seconds to about 5minutes, about 10 seconds to about 4 minutes, about 15 seconds to about3 minutes, about 20 seconds to about 2 minutes, or about 25 seconds toabout 1 minute.

In one embodiment, the subject invention provides a method for detectingsmall molecules that are biomarkers for diagnosis of a disease orcondition, or monitoring therapeutic response to specific treatments. Inspecific embodiments, the condition can be, for example, cancer, aninjury, an inflammatory disease or a neurodegenerative disease. In someembodiments, the condition can be substance abuse, psychosis,schizophrenia, Parkinson's disease, attention deficit hyperactivitydisorder (ADHD), and pain. In some embodiments, the pain is acute painor chronic pain. In some embodiments, the pain is neuropathic pain,e.g., chronic neuropathic pain.

In one embodiment, the subject invention provides a kit for detecting asmall-molecule target, comprising the CBSAzyme-based sensor. The kit canfurther comprise instructions for using the kit. In some embodiments,the kit may comprise other reagents suitable for detecting thesmall-molecule target. The reagents may include ABTS, H₂O₂, hemin, andstabilizing agents.

In one embodiment, the methods, assays and products according to thesubject invention can be used for the sensitive and accurate detectionof small-molecule targets in fields including environmental monitoring,food safety, law enforcement, medical diagnostics, and public health.

The subject invention encompasses the use of sequences having a degreeof sequence identity with the nucleic acid sequence(s) of the presentinvention. A similar sequence is taken to include a nucleotide sequencewhich may be at least 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%, 59%,60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%,74%, 75%, 76%, 77%, 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%,88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identicalto the subject sequence. Typically, the similar sequences will comprisethe same or similar secondary structure as the subject nucleic acidaptamer. In one embodiment, a similar sequence is taken to include anucleotide sequence which has one or several additions, deletions and/orsubstitutions compared with the subject sequence.

The term “about” means within an acceptable error range for theparticular value as determined by one of ordinary skill in the art,which will depend in part on how the value is measured or determined,i.e., the limitations of the measurement system. For example, “about”can mean within 1 or more than 1 standard deviation, per the practice inthe art. Alternatively, “about” can mean a range of up to 0-20%, 0 to10%, 0 to 5%, or up to 1% of a given value. Alternatively, particularlywith respect to biological systems or processes, the term can meanwithin an order of magnitude, preferably within 5-fold, and morepreferably within 2-fold, of a value. Where particular values aredescribed in the application and claims, unless otherwise stated theterm “about” meaning within an acceptable error range for the particularvalue should be assumed. In the context of compositions containingamounts of ingredients where the term “about” is used, thesecompositions contain the stated amount of the ingredient with avariation (error range) of 0-10% around the value (X±10%).

EXAMPLES Experimental Section Materials and Methods Materials.

2,2′-azinobis(3-ethylbenzthiazoline)-6-sulfonic acid (ABTS), benzocaine,caffeine, chlorpromazine HCl, cocaine HCl, diphenhydramine HCl,levamisole HCl, lidocaine HCl, methamphetamine HCl, promazine HCl,scopolamine HCl, and sucrose were purchased from Sigma-Aldrich. IHeminwas purchased from Frontier Scientific and prepared as a 5 mM stocksolution in dimethyl sulfoxide (DMSO) and stored at −20° C. before use.30% hydrogen peroxide (H₂O₂) was purchased form Fisher Scientific.Butylone, ethylone, 3,4-methylenedioxy-α-pyrrolidinobutiophenone,3,4-methylenedioxypyrovalerone, mephedrone, methedrone, methylone,4-methyl-α-pyrrolidinobutiophenone,4′-methyl-α-pyrrolidinohexanophenone, naphyrone, and pentylone werepurchased from Cayman Chemical. All synthetic cathinones were purchasedas hydrochloride salts. All other chemicals were purchased fromSigma-Aldrich unless otherwise specified. All DNA oligonucleotides werepurchased from Integrated DNA Technologies. Oligonucleotides weredissolved in PCR grade water and DNA concentrations were measured with aNanoDrop 2000 (Thermo Scientific). The names and sequences of the DNAoligonucleotides (5′-3′) are listed below, where X, Y and Z indicateselected modification sites within the spacers between GGG repeats;/5IAbRQ/represents IowaBlack RQ quencher; /3Cy5Sp/represents Cy5fluorophore.

Strand A: (SEQ ID NO: 1) 5′-AGT AAC AAC AAT CAA AAT ATG XGG YGG GT-3′Strand A0: (SEQ ID NO: 2) 5′-AGT AAC AAC AAT CAA AAT ATG TGG AGG GT-3′Strand A1: (SEQ ID NO: 3) 5′-AGT AAC AAC AAT CAA AAT ATG GGA GGG T-3′Strand A2: (SEQ ID NO: 4) 5′-AGT AAC AAC AAT CAA AAT ATG GGC GGG T-3′Strand B: (SEQ ID NO: 5) 5′-AGG GZG GGA AAT TTT GAT TGT TGT TAC T-3′Strand B0: (SEQ ID NO: 6) 5′-AGG GAC GGG AAA TTT TGA TTG TTG TTA CT-3′Strand B1: (SEQ ID NO: 7) 5′-AGG GCG GGA AAT TTT GAT TGT TGT TAC T-3′COC-5335-LF: (SEQ ID NO: 8)5′-CTC CTT CAA CGA AGT GGG TCT CCT TCA ACG AAG TGG GTC TC-3′COC-5335-SF: (SEQ ID NO: 9) 5′-GA GAC AAG GTG ACA AGG AG-3′COC-CBSAzyme-5335-22-LF: (SEQ ID NO: 10)5′-CTC CTT CAA CGA AGT GGG TCT CCT TCA ACG AAG TGG GTC TCA TGG GCG GGT-3′COC-CBSAzyme-5335-22-SF: (SEQ ID NO: 11)5′-AGG GCG GGA AGA GAC AAG GTG ACA AGG AG-3′ COC-CBSAzyme-5334-22-LF:(SEQ ID NO: 12)5′-CTC CTT CAA CGA AGT GGG TCT CCT TCA ACG AAG TGG GTC TAT GGG CGG GT-3′COC-CBSAzyme-5334-22-SF: (SEQ ID NO: 13)5′-AGG GCG GGA AAG ACA AGG TGA CAA GGA G-3′ COC-CBSAzyme-5333-22-LF:(SEQ ID NO: 14)5′-CTC CTT CAA CGA AGT GGG TCT CCT TCA ACG AAG TGG GTC ATG GGC GGG T-3′COC-CBSAzyme-5333-22-SF: (SEQ ID NO: 15)5′-AGG GCG GGA AGA CAA GGT GAC AAG GAG-3′ COC-5334-SF: (SEQ ID NO: 16)5′-AG ACA AGG TGA CAA GGA G-3′ COC-5334-LF: (SEQ ID NO: 17)5′-CTC CTT CAA CGA AGT GGG TCT CCT TCA ACG AAG TGG GTC-3′COC-CBSAzyme-5334-13-LF (AT linker): (SEQ ID NO: 18)5′-CTC CTT CAA CGA AGT GGG TCT CCT TCA ACG AAG TGG GTC TAT GGG-3′COC-CBSAzyme-5334-13-SF (AA linker): (SEQ ID NO: 19)5′-GGG TAG GGC GGG AAA GAC AAG GTG ACA AGG AG-3′COC-CBSAzyme-5334-13-LF (A linker): (SEQ ID NO: 20)5′-CTC CTT CAA CGA AGT GGG TCT CCT TCA ACG AAG TGG GTC TAG GG-3′COC-CBSAzyme-5334-13-SF (A linker): (SEQ ID NO: 21)5′-GGG TAG GGC GGG AAG ACA AGG TGA CAA GGA G-3′ SAzyme-334-LF:(SEQ ID NO: 22) 5′-GTC TCC TTC AAC GAA GTG GGT CTA GGG-3′ SAzyme-334-SF:(SEQ ID NO: 23) 5′-GGG TAG GGC GGG AAA GAC AAG GTG AC-3′MDPV-Binding Aptamer: (SEQ ID NO: 24)5′-CTT ACG ACT CAG GCA TTT TGC CGG GTA ACG AAG TTA CTG TCG TAA G-3′MDPV-6335-LF: (SEQ ID NO: 25)5′-TAC GAC TCA GGC TTT GCC GGG TAT GAC TCA GGC TTT GCC GGG TAA C-3′MDPV-6335-SF: (SEQ ID NO: 26) 5′-G TTA CTG TCT TAC TGT CGT A-3′MDPV-6335-SF-FQ: (SEQ ID NO: 27)5′-/5IAbRQ/G TTA CTG TCT TAC TGT CGT A/3Cy5Sp/-3′ MDPV-CBSAzyme-LF:(SEQ ID NO: 28)5′-TAC GAC TCA GGC TTT GCC GGG TAT GAC TCA GGC TTT GCC GGG TAA CAG GG-3′MDPV-CBSAzyme-SF: (SEQ ID NO: 29)5′-GGG TAG GGC GGG AAG TTA CTG TCT TAC TGT CGT A-3′LF of parent split MDPV aptamer (1): (SEQ ID NO: 30)5′-TAC GAC TCA GGC TTT GCC GGG TA-3′SF of parent split MDPV aptamer (1): (SEQ ID NO: 31) 5′-T ACT GTC GTA-3′LF of parent split MDPV aptamer (2): (SEQ ID NO: 32)5′-GAC TCA GGC TTT GCC GGG TAA C-3′ SF of parent split MDPV aptamer (2):(SEQ ID NO: 33) 5′-GTT ACT GTC-3′ COC-5333-LF: (SEQ ID NO: 34)5′-CTC CTT CAA CGA AGT GGG TCT CCT TCA ACG AAG TGG GTC-3′ COC-5333 SF:(SEQ ID NO: 35) 5′-GAC AAG GTG ACA AGG AG-3′Segment 1 of 2:2 split DNAzyme: (SEQ ID NO: 36) 5′-GXGGYGGG-3′Segment 2 of 2:2 split DNAzyme: (SEQ ID NO: 37) 5′-GGGZGGG-3′Split DNAzyme segment from A0: (SEQ ID NO: 38) 5′-GTGGAGGGT-3′Split DNAzyme segment from A1: (SEQ ID NO: 39) 5′-GGGAGGGT-3′Split DNAzyme segment from A2: (SEQ ID NO: 40) 5′-GGGCGGGT-3′Split DNAzyme segment from B0: (SEQ ID NO: 41) 5′-AGGGACGGG-3′Split DNAzyme segment from B1: (SEQ ID NO: 42) 5′-AGGGCGGG-3′. DNAzyme:(SEQ ID NO: 43) 5′-GGG CGG GTA GGG CGG G-3′Segment 1 of 1:3 split DNAzyme: (SEQ ID NO: 44) 5′-GGG-3′Segment 2 of 1:3 split DNAzyme: (SEQ ID NO: 45) 5′-GGG TAG GGC GGG-3′

Determination of Split DNAzyme Activity Utilizing Duplex DNA.

Duplex-DNA-DNAzyme conjugates were prepared by mixing strands A0 and B0,A1 and B0, A2 and B0, or A2 and B1 (0.25 μM final concentration for eachstrand) in 40 mM HEPES buffer (pH 7.0) containing 50 mM KCl, 0.05% (v/v)Triton X-100, and 1% (v/v) DMSO. Freshly prepared hemin was then addedto a final concentration of 1 μM, and the solution was incubated for 30minutes at room temperature. The mixture was then added to a 384-wellplate, and H₂O₂ (final concentration 2 mM) and ABTS (final concentration1.5 mM) were added to initiate the reaction. The absorption intensity atλ=418 nm (ABTS^(•+)) was recorded every minute using a Tecan InfiniteM1000 PRO microplate reader. The concentration of ABTS^(•+) wascalculated based on the extinction coefficient of ABTS^(•+) at λ=418 nm(c=36000 M⁻¹ cm⁻¹). The concentration of ABTS^(•+) was plotted as afunction of time and the initial reaction rates (V₀) were determined bycalculating the slope of the linear portion of the plot. This parameterwas used to evaluate the catalytic activity of the split DNAzymes.

Optimization of KCl and NaCl Concentration.

To maximize the signal gain from the CBSAzymes, the bufferconcentrations of KCl and NaCl were optimized using a two-factor,12-level uniform design. A combination of 12 different KCl and NaClconcentrations was tested. 5 μL of varying concentrations of 10×KCl andNaCl were mixed with 4 μL of HEPES buffer (final concentration: 40 mM,pH 7.0), 13 μL of deionized water (DI), and 2.5 μL of each CBSAzymefragment (final concentration of each strand: 1 μM). 5 μL of cocaine orMDPV (final concentration: 250 μM) was then added to the mixture and thesolution was incubated for 30 minutes. Afterwards, 0.5 μL hemin (finalconcentration 1 μM) and 2.5 μL Triton X-100 (final concentration 0.05%)were added to the mixture, and the solution was incubated for another 30minutes. The solution was subsequently transferred into a well of a384-well plate and then a 10 μL of a substrate solution (finalconcentrations: 2 mM H₂O₂, 1.5 mM ABTS and 40 mM HEPES) was added. Theabsorbance at 418 nm was monitored every minute using a Tecan InfiniteM1000 PRO microplate reader. The combination of salt concentrations thatachieved the highest signal gain was used for subsequent experiments.

Determination of CBSAzyme Kinetics.

The following experiments were performed at room temperature. TheMichaelis-Menten constant (K_(M)) and turnover number (k_(cat)) of theCBSAzyme were determined by varying the concentration of ABTS.Specifically, 1 μM of each fragment was mixed with 250 μM cocaine in 40mM HEPES (pH 7.0) containing 0.05% Triton X-100, 1% DMSO and optimalconcentrations of KCl and NaCl as determined above. This mixture wasincubated for 30 minutes and then 1 μM hemin was added. After 30 minutesof incubation, the sample was loaded into a well of a 384-wellmicroplate and the enzymatic reaction was initiated by the addition of 2mM H₂O₂ and various concentrations of ABTS (final concentration: 0.1,0.5, 1.0, 1.5, 2.0, 4.0, or 6.0 mM). The absorbance at 418 nm wasmonitored every minute using a Tecan Infinite M1000 PRO microplatereader and the corresponding absorbance values were converted toconcentration using the extinction coefficient of ABTS^(•+) at λ=418 nm(c=36000 M⁻¹ cm⁻¹). The concentration of ABTS^(•+) was plotted againsttime, and the initial reaction rates were determined by calculating theslope of the linear portion of the plot. To determine K_(M) and maximalvelocity (V_(max)), the initial reaction rate was plotted against theconcentration of ABTS, and that plot was fitted with theMichaelis-Menten equation²⁸ using nonlinear regression.

Colorimetric Detection of Cocaine.

Detection was performed by incubating cocaine (final concentrations: 0,0.1, 0.3, 1, 3, 10, 30, 100, 300, or 1,000 μM) with 1 μM each ofCOC-CBSAzyme-SF and COC-CBSAzyme-LF in 40 mM HEPES (pH 7.0) with 1 μMhemin, 1 mM KCl, 30 mM NaCl, 0.05% Triton X-100, and 1% DMSO for 15minutes at room temperature. The sample was then loaded into a well of a384-well microplate, and the enzymatic reaction was initiated by theaddition of 2 mM H₂O₂ and 1.5 mM ABTS (final concentrations). Theabsorbance at 418 nm was monitored every minute using a Tecan InfiniteM1000 PRO microplate reader.

Fluorescence Assay for MDPV.

The long fragment and fluorophore-quencher-modified short fragment ofMDPV-6335 (final concentration of each: 1 μM) was mixed with 40 mM HEPESbuffer (pH 7.0) containing 7 mM KCl and 77 mM NaCl. Variousconcentrations of MDPV (final concentrations: 1, 3, 10, 30, 100, 300,1000, or 3000 μM) were added into the mixture, and the solution wasincubated for 30 minutes at room temperature. The total reaction volumewas 100 μL. Each reaction was loaded into a well of a 96-well microplateand the fluorescence intensity at 668 nm was measured using a TecanInfinite M1000 PRO microplate reader with excitation at 648 nm. Thesignal gain was calculated by (F−F₀)/F₀×100%, where F₀ is thefluorescence of MDPV-6335 mixture without MDPV, and F is thefluorescence of MDPV-6335 mixtures with different concentration of MDPV.Signal gain was plotted against the employed MDPV concentration, and theplot was fitted with the Hill equation using Origin 2017 software tocalculate the Hill coefficient (n_(H)) and MDPV concentration producinghalf occupancy (K_(1/2)).

Colorimetric Detection of MDPV.

Briefly, 1 μM each of MDPV-CBSAzyme-SF and MDPV-CBSAzyme-LF wereincubated with MDPV (final concentrations: 0, 0.1, 0.3, 1, 3, 10, 30,100, 300, 1,000 μM) in 40 mM HEPES (pH 7.0) with 1 μM hemin, 7 mM KCl,77 mM NaCl, 0.05% Triton X-100, and 1% DMSO. This mixture was incubatedfor 15 minutes at room temperature. The sample was then loaded into awell of a 384-well microplate and the enzymatic reaction was initiatedby the addition of 2 mM H₂O₂ and 1.5 mM ABTS (final concentrations). Theabsorbance at 418 nm was monitored every minute using a Tecan InfiniteM1000 PRO microplate reader.

Circular Dichroism Measurements.

Circular dichroism experiments, including sample preparation, wereperformed at room temperature. For cocaine, 1 μM of each CBSAzymefragment was incubated with or without 250 μM cocaine in 40 mM HEPESbuffer (pH 7.0) containing 1% DMSO, and optimal KCl and NaClconcentrations (listed under respective circular dichroism figures inESI) for 30 minutes. Hemin (final concentration 1 μM) or DMSO (forcontrol experiments, final concentration 1% (v/v) for controlexperiments) was added, and the solution was incubated for another 30minutes. The samples (300 μL) were transferred into a 1 cm quartzcuvette (Hellma Analytics) to perform circular dichroism measurementsusing a Jasco J-815 circular dichroism spectropolarimeter with scanrange: 235 to 300 nm, scanning speed: 50 nm/min, sensitivity: 100 mdeg,response time: 1 s, bandwidth: 1 nm, total scans: 6. For MDPV, hemin(final concentration 1 μM) or DMSO (for control experiments, finalconcentration 1% (v/v)) was added to 1 μM of each MDPV-CBSAzyme fragmentwith or without 200 μM MDPV in 40 mM HEPES buffer (pH 7.0) containing 1%DMSO, 7 mM KCl, and 77 mM NaCl. The mixture was incubated overnight atroom temperature. The circular dichroism spectra were recorded with scanrange: 220 to 300 nm, scanning speed: 50 nm/min, sensitivity: 5 mdeg,response time: 8 s, bandwidth: 1 nm, total scans: 6. For data analysis,all circular dichroism spectra were averaged and then corrected bysubtracting the circular dichroism spectra of the reaction buffer withor without drug.

Example 1—Engineering a Split DNAzyme with High Activity

Peroxidase-mimicking DNAzymes are G-quadruplex-structuredoligonucleotides that can perform a catalytic reaction similar to thatof horseradish peroxidase. These DNAzymes can be split into twofragments, either in a symmetrical or asymmetrical fashion. For example,Willner et al. have split a single-stranded DNAzyme into two symmetricalsegments, with each strand containing two GGG repeats. This is known asthe 2:2 split mode. The split DNAzyme segments can be reconstituted withhemin when they are brought into close proximity to form a layeredG-quadruplex complex that exhibits peroxidase-like activity, catalyzingthe oxidation of colorless2,2′-azinobis(3-ethylbenzothiozoline)-6-sulfonic acid (ABTS) into darkgreen ABTS^(•+) by H₂O₂. However, the catalytic activity of this splitDNAzyme is low. Its activity could be improved by altering the spacersbetween the guanine triplets.

To test this hypothesis, pairs of duplex DNA-DNAzyme conjugates weredesigned with each fragment containing one segment of the 2:2 splitDNAzyme (FIG. 1A), the activity of the original split DNAzyme was firsttested (FIG. 1B, A0-B0) using the rate of ABTS^(•+) produced as abenchmark. A reaction rate of 14.5 nM/s was observed for this nativesplit DNAzyme (FIG. 1B). A derivative of strand A0 was then synthesizedby removing the thymine at position X to generate strand A1. Theassembled A1-B0 split DNAzyme had a higher reaction rate (19.0 nM/s)(FIG. 2A) than the original split DNAzyme (FIG. 1B). This indicated thatthe thymine bulge originally present within the GGG repeat may bedisruptive to the assembly of the split DNAzyme. The adenine at positionY in strand A1 was further replaced with the less bulky cytosine togenerate strand A2, and the reaction rate of the assembled A2-B0 splitDNAzyme (FIG. 1B) further increased to 20.9 nM/s (FIG. 2A). Thisimprovement in activity may be attributed to the lower steric hindranceproduced by cytosine relative to adenine, which provides betterflexibility for DNAzyme assembly and thus results in higher catalyticactivity. Finally, a derivative of strand B0 was synthesized, in whichthe adenine at position Z was removed from the adenine-cytosine spacerto form strand B1. The assembled A2-B1 split DNAzyme (FIG. 1C) had amuch higher activity (27.3 nM/s) compared to A2-B0 (FIG. 2B). Theseresults showed that shorter spacers promoted formation of a more compactG-quadruplex structure that boosts catalytic activity.

Example 2—Designing CBSA-DNAzyme Conjugates

A cocaine-binding CBSA (COC-5335) (FIG. 3A) containing two binding siteshas been previously described. The cocaine-binding CBSA (COC-5335) wasfar more responsive to its target than its parent split aptamer with asingle binding domain. Using this CBSA, ultra-sensitive, one-stepdetection of cocaine was achieved within fifteen minutes. A CBSA-DNAzymeconjugate was developed for rapid naked-eye cocaine detection usingCOC-5335 and the optimized split DNAzyme studied above.

Specifically, the DNAzyme segment from Strand A2 (5′-GGGCGGGT-3′) to the3′ terminus of the long fragment of COC-5335 was attached via an A-Tdinucleotide linker, and the DNAzyme segment from Strand B1(5′-AGGGCGGG-3′) to the 5′ terminus of the short fragment of COC-5335was attached via an A-A dinucleotide linker (FIG. 3B). This conjugatewas termed CBSAzyme-5335-22. In the absence of cocaine, these twoCBSAzyme fragments are separated and the split DNAzyme remainsunassembled and incapable of oxidizing ABTS; thus, the solution remainsclear (FIG. 4, left). In the presence of cocaine, both fragmentsassemble, bringing the two DNAzyme segments into close proximity. Theassembled DNAzyme module accommodates hemin within its G-quadruplex,allowing for H₂O₂-mediated oxidation of ABTS to ABTS^(•+), therebyrapidly turning the solution from clear to dark green (FIG. 4, right).

Example 3—Effect of CBSA Stem Length on CBSAzyme Performance

250 nM of the short and long fragments of CBSAzyme-5335-22 were mixed inreaction buffer in the presence or absence of 250 μM cocaine. After 10minutes, the absorption of ABTS^(•+) at 418 nm greatly increased in thepresence of cocaine. In contrast, the absorbance intensity only slightlyincreased in the absence of cocaine, most likely due to nonspecificbackground assembly of the CBSAzyme (FIG. 5A).

To evaluate whether reducing thermostability can mitigate backgroundassembly of the CBSAzyme and improve signal gain, derivatives ofCBSAzyme-5335-22 were synthesized by truncating one or two base pairsfrom the termini of the CBSA fragments closest to the DNAzyme segmentsto generate CBSAzyme-5334-22 and CBSAzyme-5333-22, respectively. Theperformance of these derivatives was then tested under their optimizedreaction conditions. CBSAzyme-5334-22 achieved lower background assemblywhile maintaining excellent catalytic activity (FIG. 5B), producing ahigher signal gain than CBSAzyme-5335-22. CBSAzyme-5333-22 (FIG. 5C)showed slightly decreased activity while having the same backgroundsignal as CBSAzyme-5334-22, demonstrating that further decreases in thethermostability of the CBSAzyme are deleterious for assay performance.CBSAzyme-5334-22 was used for subsequent experiments.

Example 4—Effects of the DNAzyme Split Mode on CBSAzyme Performance

A large background signal was observed in the absence of cocaine withCBSAzyme-5334-22. Since further destabilization of the CBSA moduledecreased the signal gain, the DNAzyme module was destabilized as analternative means to reduce the background signal. Studies have shownthat symmetrically split DNAzymes can more easily self-assemble thanthose that are asymmetrically split. The DNAzyme was therefore split ina 1:3 mode, with one segment having three GGG repeats and the other onlyhaving one, and joined these fragments to the CBSA. Specifically, thethree-repeat segment was coupled to the short fragment of the CBSA(COC-5334-SF), and the single-repeat segment was coupled onto the longfragment (COC-5334-LF), thereby generating CBSAzyme-5334-13 (FIG. 6A).

The performances of CBSAzyme-5334-13 and CBSAzyme-5334-22 were comparedin the absence and presence of 250 μM cocaine, and different levels ofactivity and background were observed (FIGS. 6B and C). CBSAzyme-5334-13demonstrated much lower background assembly relative to CBSAzyme-5334-22in the absence of cocaine, most likely due to the low stability of 1:3split DNAzymes. The 1:3 split CBSAzyme also had a higher level ofactivity in the presence of cocaine relative to the 2:2 split CBSAzyme.The Michaelis-Menten constant (K_(M)) and turnover number (k_(cat)) werefurther determined for both CBSAzymes. In the absence of cocaine,CBSAzyme-5334-22 had a slightly lower K_(M) and higher k_(cat), than the1:3 split CBSAzyme, which explains its higher background signal. In thepresence of cocaine, CBSAzyme-5334-13 had a much lower K_(M) and higherk_(cat), than the 2:2 split CBSAzyme (FIGS. 7A and B), consistent withthe observed higher catalytic activity.

The target-induced conformational change of CBSAzyme-5334-13 was thencharacterized using circular dichroism (FIG. 8). In the presence of onlythe CBSAzyme, two distinct peaks: a negative peak at 245 nm and a broadpositive peak spanning from approximately 260 to 280 nm were observed.The broad positive peak possibly encompasses two merged peaks, whereinone peak at 260 nm arises from a parallel G-quadruplex structure andanother at approximately 270 nm originates from a duplex DNA structure.Furthermore, both the G-quadruplex and duplex DNA structures maycontribute to the presence of the negative peak at 245 nm. When 1 μMhemin was added, no measurable change occurred in the shape or size ofthese peaks, indicating that hemin itself cannot induce assembly of theCBSA or DNAzyme. The ellipticity increased at 270 nm when only cocainewas added, demonstrating target-induced CBSA assembly. Further additionof hemin induced a peak shift from 270 to 265 nm with an accompanyingincrease in the ellipticity of the peak, which likely corresponded tofull assembly of the CBSA and DNAzyme modules. This shows that althoughcocaine can assemble both CBSA fragments, hemin is required for splitDNAzyme assembly.

As a control, the conformational changes of CBSAzyme-5334-22 weremonitored (FIG. 9). The circular dichroism spectrum for this CBSAzymeresembled that of CBSAzyme-5334-13. Hemin alone again induced noobservable change in the size or shape of the spectra. In the presenceof cocaine alone, the ellipticity of the peaks at 263 nm and 270 nmnotably increased, but the subsequent addition of hemin did not causeany meaningful further change in the spectra. These results indicatethat the split DNAzyme segments can readily assemble even without hemin,supporting the notion that the 2:2 split DNAzyme has a strong tendencyto self-assemble. This likely explains the large improvement inbackground signal observed when using the 1:3 split mode.

Example 5—Optimization of the CBSAzyme Linker

To further optimize the signal gain produced by the CBSAzyme, the impactof the linker length between the CBSA and DNAzyme modules ontarget-induced assembly and catalytic response was investigated. Avariant of the long fragment of CBSAzyme-5334-13 was first synthesized,in which the linker was shortened from AT to A and the assay with thecorresponding short fragment was then performed in the presence andabsence of 250 μM cocaine.

In comparison to the original construct, which achieved acocaine-induced signal gain of 4.50 (FIG. 10A), shortening the linker ofthe long fragment yielded improved CBSAzyme performance, with a signalgain of 5.27 (FIG. 10B). The linker of the short fragment ofCBSAzyme-5334-13 was then modified by replacing the original AA linkerwith a single A, and this variant's performance was tested inconjunction with the optimized long fragment. Shortening this linker haddeleterious effects on DNAzyme performance (signal gain=2.83) (FIG.10C), and the original linker of the short fragment (AA) was thereforeretained. The optimized A/AA linker combination favors assembly of boththe CBSA and the split DNAzyme. Circular dichroism analysis of theassembled split DNAzyme confirmed that it retained a parallelG-quadruplex structure in the presence of hemin and cocaine (FIG. 11),corresponding to its high DNAzyme activity.

Example 6—Colorimetric Detection of Cocaine with the CBSAzyme

The optimized construct, termed COC-CBSAzyme, was employed for thevisual detection of cocaine. A series of control experiments verifiedthat signal is only obtained in the presence of cocaine (FIG. 12A). Nosignal was observed with H₂O₂ and ABTS alone; when hemin was added, ABTSwas slowly oxidized by H₂O₂ to produce a minor increase in absorbance at418 nm. Further addition of the long fragment did not produce any signalchange, but addition of the short fragment with hemin in the absence oflong fragment produced low levels of background. This is probablybecause the short fragment contains three guanine triplets, which mayform intra- or intermolecular G-quadruplexes that can accommodate heminto catalyze the oxidation of ABTS. When both fragments were included inthe same mixture without target, low levels of background signalidentical to that obtained with the short fragment alone were observed,indicating that the fragments only underwent minimal assembly in theabsence of target. The addition of 250 μM cocaine promoted assembly ofthe split DNAzyme, yielding a large change in absorbance (FIG. 12A) andrapid development of a dark green color within 15 minutes (FIG. 13). Todetermine the optimum concentrations of short and long fragment, theCOC-CBSAzyme-based assay was performed with varying concentrations ofshort fragment (0.2 to 2 μM) while keeping the concentration of the longfragment at 1 μM and vice versa. The results show that 1 μM of eachfragment produces the highest signal gain (FIG. 14).

The performance of the assay was then determined for the visualdetection of cocaine. In the absence of cocaine, the color of thesolution did not change, even after 30 minutes of reaction. However, inthe presence of cocaine, the absorbance of ABTS^(•+) gradually increasedand the solution became green over time (FIG. 15). Cocaine atconcentrations as low as 10 μM was detected with the naked eye withinonly 5 minutes, and the color intensity of the solution was proportionalto the concentration of cocaine (FIG. 15). A microplate reader was usedto generate a calibration curve using the absorbance of ABTS^(•+) at 418nm after 15 minutes of reaction and obtained a measurable limit ofdetection of 1 μM with a linear range of 0 to 100 μM (FIGS. 12B and C).

The performance of the CBSAzyme was compared to a DNAzyme-linked splitaptamer with a single binding site. This variant was synthesized bytruncating the 5-bp double-stranded duplex and the adjacent binding sitefrom the 5′ end of the long fragment of COC-CBSAzyme to form SAzyme-334(FIG. 16A). Whereas a clear color change was observed with COC-CBSAzymeat cocaine concentrations as low as 10 μM after only 2 minutes, a colorchange with SAzyme-334 was not observed at cocaine concentrations lowerthan 300 μM, even after 15 minutes (FIG. 16B). This demonstrates the farlower target responsiveness of the single-site SAzyme-334 constructrelative to the COC-CBSAzyme.

The specificity of the assay was further tested against 250 μMconcentrations of various interferent drugs, including chlorpromazine,diphenhydramine, promazine, and scopolamine, as well as common cuttingagents found in street samples such as caffeine, levamisole, lidocaine,and sucrose (FIG. 17A). No cross-reactivity to scopolamine and sucrose,and minimal cross-reactivity to caffeine, chlorpromazine, promazine, andlevamisole at this concentration was observed. However, moderatecross-reactivity was observed for diphenhydramine and lidocaine (FIGS.17B and C). This was expected, given that the cocaine-binding aptamerhas previously-reported cross-reactivity to these molecules.

Example 7—Demonstrating Generality of the CBSAzyme Assay Format forDetection of MDPV

MDPV is a member of the family of designer drugs known as syntheticcathinones—a class of drugs for which assay development lags well behindthe emergence of new molecules into the market. An MDPV-binding CBSA wasgenerated based on an isolated three-way-junction-structured DNA aptamer(FIG. 18A, I) that binds to MDPV with a K_(D) of 6 μM. Specifically, twodifferent parent split aptamer pairs (FIG. 18A, II) was derived with asingle binding pocket from the MDPV-binding aptamer, in which the GAAloop from stem 3 was removed and the number of base-pairs in all stemswas decreased. Stem 1 of one parent split aptamer was then connected tostem 3 of the second via a single thymine linker on each strand to forma MDPV-binding CBSA, MDPV-6335 (FIG. 18A, III).

To determine the binding affinity and cooperativity of MDPV-6335, theshort fragment was modified with a 5′ IowaBlack RQ quencher and a 3′ Cy5fluorophore (FIG. 18B). In the absence of MDPV, the two fragments remainseparate, with the fluorophore-quencher pair of the short fragmentremaining in close proximity to each other and thus yielding nofluorescence signal (FIG. 18B, left). When the target is added, the CBSAassembles to form a rigid target-CBSA complex, separating thefluorophore-quencher pair and producing a large fluorescence signal(FIG. 18B, right). This fluorophore-quencher-modified version ofMDPV-6335 was used to generate a binding curve for MDPV concentrationsranging from 0-3,000 μM, and the resulting curve was fitted with theHill equation (FIG. 18C). MDPV-6335 had a binding affinity (K_(1/2),target concentration producing half occupancy) of 140.6 μM with acooperativity (n_(H)) of 1.8, which shows the high degree of targetbinding cooperativity.

A MDPV-binding CBSAzyme was then generated by incorporating theoptimized DNAzyme and linker sequences described above into the shortand long fragments of MDPV-6335 to form MDPV-CBSAzyme (FIG. 19A). Theperformance of this CBSAzyme was tested and a high signal gain wasobtained in the presence of MDPV after 15 minutes (FIG. 19B). Circulardichroism was also used to confirm the structure and target-inducedconformational changes of MDPV-CBSAzyme (FIG. 20). For the CBSAzymealone, two peaks were observed: a negative peak at 241 nm and a broadpositive peak with a maximum at 275 nm and a shoulder at 260 nm. Thepeaks at 260 nm and 275 nm correspond to background assembly of theG-quadruplex-structured DNAzyme and duplex-structured CBSA,respectively, while the peak at 241 nm represents assembly of bothmodules. With the addition of hemin, the spectra did not change,indicating that hemin was unable to assemble either the CBSA or thesplit DNAzyme. With the addition of MDPV alone, a slight increase wasobserved in the 275 nm positive peak, indicating low levels oftarget-induced CBSA assembly. When both hemin and MDPV were added, thepositive peaks at 260 nm and 275 nm increased, confirming that bothhemin and MDPV are required for efficient assembly.

This MDPV-CBSAzyme was used for visual detection of MDPV. Controlexperiments confirmed that the CBSAzyme only produces a target-relatedsignal (FIGS. 21A and 22). No background signal was observed with thereaction buffer and hemin, or with the long fragment and hemin. As withthe cocaine assay, a very small absorbance signal was observed with themixture of short fragment and hemin, probably due to the fact that itsthree guanine triplets form inter- or intramolecular G-quadruplexes thatcan accommodate hemin to facilitate ABTS oxidation. Similar low levelsof absorbance signal was observed when both long and short fragmentswere combined with hemin in the absence of target, but no visible colorchange in the solution occurred even after 30 minutes of reaction. WhenMDPV was added, the absorbance of ABTS^(•+) gradually increased (FIG.21A) and the solution rapidly developed a dark green color over 15minutes (FIG. 22). A calibration curve was then generated using theabsorbance ABTS^(•+) at 418 nm after 15 minutes and obtained a detectionlimit of 3 μM MDPV with a linear range from 0 to 100 μM (FIGS. 21B andC). Importantly, MDPV concentrations as low as 30 μM were able to bedetected by naked eye within 5 minutes (FIG. 23).

The MDPV-CBSAzyme can be used to visually detect a variety of syntheticcathinones. Specifically, The CBSAzyme-based assay was challengedagainst 250 μM concentrations of 11 different synthetic cathinones,including MDPV, methylone, pentylone,3,4-methylenedioxy-α-pyrrolidinobutiophenone (MDPBP), mephedrone,4-methyl-α-pyrrolidinobutiophenone (MPBP),4′-methyl-α-pyrrolidinohexanophenone (MPHP), naphyrone, methedrone,ethylone, and butylone. As expected, a dark green color (FIG. 24A) and ahigh signal gain (FIG. 25) were observed after 15 minutes for all testedsynthetic cathinones, indicating that MDPV-CBSAzyme retains excellentcross-reactivity to structurally-similar synthetic cathinone analogs. Inaddition, the CBSAzyme-based assay retained excellent specificityagainst interferents, as no measurable signal was observed with 250 μMconcentrations of common cutting agents and illicit drugs such ascaffeine, benzocaine, lidocaine, sucrose and methamphetamine (FIGS. 24Band 26). Based on the success of CBSAzyme-based assays for colorimetriccocaine and MDPV detection, this assay can be generalized to detect anysmall molecule in a simple, label- and instrument-free manner.

Example 8—Detecting Cocaine and MDPV in Biological Samples Using theCBSAzyme-Based Sensor

To demonstrate the visual detection of cocaine in a biological sample,COC-CBSAzyme (LF: SEQ ID NO: 20; SF: SEQ ID NO: 19) was used to detectcocaine at different concentrations of cocaine (COC) (0-1000 μM) in 10%saliva (FIG. 27) and 10% urine (FIG. 28) samples, respectively. Thecolor of the cocaine-containing samples progressively changes over time,while the color of the cocaine-free sample only changed slightly (FIGS.27 left and 28 left). Cocaine at concentrations of 10 μM and higher canbe detected within one minute. A calibration curve was generated using0-1000 μM cocaine after five minutes of reaction in 10% saliva (FIG. 27right) and 10% urine (FIG. 28 right). The limit of visual detection is 3μM cocaine.

To demonstrate the visual detection of MDPV in a biological sample,MDPV-CBSAzyme (LF: SEQ ID NO: 28; SF: SEQ ID NO: 29) was used to detectMDPV at different concentrations of MDPV (0-1000 μM) in 50% saliva (FIG.29) and 50% urine (FIG. 30) samples, respectively. The color of theMDPV-containing samples progressively changes over time, while the colorof the MDPV-free samples shows no obvious changes (FIGS. 29 left and 30left). MDPV at concentrations of 30 μM and higher can be detected withinfive minute in both 50% saliva and 50% urine samples. A calibrationcurve was generated using 0-1000 μM MDPV after 15 minutes of reaction in50% vsaliva (FIG. 29 right) and 50% urine (FIG. 30 right). The limit ofvisual detection is 30 μM MDPV.

What is claimed is:
 1. A cooperative binding split aptamer (CBSA)-DNAzyme conjugate (CBSAzyme) comprising a first fragment and a second fragment, the first fragment comprising a first segment of a split DNAzyme and a long fragment of a CBSA, and the second fragment comprising a second segment of the split DNAzyme and a short fragment of the CBSA, the first and second segments of the split DNAzyme being split from a DNAzyme, the short and long fragments of the CBSA assembling upon binding of a small-molecule target.
 2. The CBSAzyme according to claim 1, the long fragment of the CBSA being linked to the first segment of the DNAzyme via a first linker, and the short fragment of the CBSA being grafted to the second segment of the DNAzyme via a second linker.
 3. The CBSAzyme according to claim 2, the first and second linkers are independently selected from A, C, T, AA, AC, AT, CC, CA, CT, TA, TC, and TT.
 4. The CBSAzyme according to claim 1, the CBSA being selected from i) COC-5335 having a long fragment of SEQ ID NO: 8 and a short fragment of SEQ ID NO: 9; ii) COC-5334 having a long fragment of SEQ ID NO: 17 and a short fragment of SEQ ID NO: 16; iii) COC-5333 a long fragment of SEQ ID NO: 34 and a short fragment of SEQ ID NO: 35; and iv) MDPV-6335 a long fragment of SEQ ID NO: 25 and a short fragment of SEQ ID NO:
 26. 5. The CBSAzyme according to claim 1, the DNAzyme having a sequence of SEQ ID NO: 43 or a sequence sharing at least 90% identity thereof.
 6. The CBSAzyme according to claim 1, the DNAzyme being split in a 1:3 or 2:2 mode.
 7. The CBSAzyme according to claim 1, which is selected from i) COC-CBSAzyme-5335-22 having a first fragment of SEQ ID NO: 10 and a second fragment of SEQ ID NO: 11; ii) COC-CBSAzyme-5334-22 having a first fragment of SEQ ID NO: 12 and a second fragment of SEQ ID NO: 13; iii)) COC-CBSAzyme-5333-22 having a first fragment of SEQ ID NO: 14 and a second fragment of SEQ ID NO: 15; iv) COC-CBSAzyme-5334-13 (AT/AA) having a first fragment of SEQ ID NO: 18 and a second fragment of SEQ ID NO: 19; v) COC-CBSAzyme-5334-13 (A/AA) having a first fragment of SEQ ID NO: 20 and a second fragment of SEQ ID NO: 19; vi) COC-CBSAzyme-5334-13 (A/A) having a first fragment of SEQ ID NO: 20 and a second fragment of SEQ ID NO: 21; and vii) MDPV-CBSAzyme having a first fragment of SEQ ID NO: 28 and a second fragment of SEQ ID NO:
 29. 8. A method for visually detecting cocaine in a sample comprising contacting the sample with a CBSAzyme-based sensor comprising a CBSAzyme selective for cocaine and a signal reporter, and detecting cocaine in the sample by determining whether a signal occurs upon cocaine binding to the CBSAzyme, the signal being a color change, which is indicative of the presence of cocaine in the sample.
 9. The method according to claim 8, the sample being a biological sample or an environmental sample.
 10. The method according to claim 8, the sample being a seized sample.
 11. The method according to claim 9, the biological sample being selected from urine and saliva.
 12. The method according to claim 8, the signal reporter being a mixture of a peroxidase substrate and H₂O₂.
 13. The method according to claim 12, the peroxidase substrate being 2,2′-azinobis(3-ethylbenzthiazo-line)-6-sulfonic acid (ABTS).
 14. The method according to claim 8, the CBSAzyme being a COC-CBSAzyme selected from i) COC-CBSAzyme-5335-22 having a first fragment of SEQ ID NO: 10 and a second fragment of SEQ ID NO: 11; ii) COC-CBSAzyme-5334-22 having a first fragment of SEQ ID NO: 12 and a second fragment of SEQ ID NO: 13; iii)) COC-CBSAzyme-5333-22 having a first fragment of SEQ ID NO: 14 and a second fragment of SEQ ID NO: 15; iv) COC-CBSAzyme-5334-13 (AT/AA) having a first fragment of SEQ ID NO: 18 and a second fragment of SEQ ID NO: 19; v) COC-CBSAzyme-5334-13 (A/AA) having a first fragment of SEQ ID NO: 20 and a second fragment of SEQ ID NO: 19; and vi) COC-CBSAzyme-5334-13 (A/A) having a first fragment of SEQ ID NO: 20 and a second fragment of SEQ ID NO:
 21. 15. A method for visually detecting one or more synthetic cathinones in a sample, the method comprising contacting the sample with a CBSAzyme-based sensor comprising a signal reporter and a CBSAzyme binding to one or more synthetic cathinones, and detecting one or more synthetic cathinones in the sample by determining whether a signal occurs upon one or more synthetic cathinones binding to the CBSAzyme, the signal being a color change, which is indicative of the presence of one or more synthetic cathinones in the sample, the synthetic cathinone having a core structure of:

R₁ and R₂, are each independently selected from the group consisting of hydrogen, alkyl, alkoxy, and hydroxylalkyl; or R₁ and R₂, taken together with the carbon atoms to which they are attached, form a substituted or unsubstituted 5- or 6-membered homocyclic or heterocyclic ring; R₃ is hydrogen, or alkyl; R₄, and R₅ are each independently selected from the group consisting of hydrogen, and alkyl; or R₄ and R₅, taken together with the nitrogen atom to which they are attached, form a substituted or unsubstituted 5- or 6-membered heterocyclic ring; and R₆ is hydrogen, or alkyl.
 16. The method according to claim 15, the sample being a seized sample or a biological sample selected from urine and saliva.
 17. The method according to claim 15, the signal reporter being a mixture of a peroxidase substrate and H₂O₂.
 18. The method according to claim 17, the peroxidase substrate being 2,2′-azinobis(3-ethylbenzthiazo-line)-6-sulfonic acid (ABTS).
 19. The method according to claim 15, the one or more synthetic cathinones being selected from 3,4-methylenedioxypyrovalerone (MDPV), penthylone, mephedrone, naphyrone, MDPBP, methylone, methedrone, ethylone, butylone, 4′-methyl-α-pyrrolidinohexanophenone (MPHP), and 4-methyl-α-pyrrolidinobutiophenone (MEPBP).
 20. The method according to claim 15, the CBSAzyme comprising a first fragment having a sequence of SEQ ID NO: 28, and a second fragment having a sequence of SEQ ID NO:
 29. 